ACTIVE SCANNING vs. ACTIVE SEARCHING
Since we’re discussing perception-type tests, let’s swap from passive perception tests to active perception tests for a little coda of sorts.
When it comes to active perception tests, there is a distinction between what I’m going to call searching (tossing a room, feeling for hidden seams on the wall, running your fingers under the edge of a table) and scanning (peering out into the darkness beyond your campfire, doing a manual spectrum-analysis with the ship’s sensors, holding your breath and listening intently).
This distinction between searching and scanning is one that I find most games either struggle with or ignore entirely, and I think this also reflects common GMing practice. I didn’t fully grok the distinction myself until it got repeatedly pushed in my face running the Tomb of Horrors: Trapped in a cycle of paranoia and terror, the players wanted to search for traps without touching anything (because touching it could trigger the very disaster they were trying to avoid).
In combination with passive perception, this essentially creates three tiers of perception:
- Not actively looking. (Passive Perception)
- Actively looking, but not physically interacting. (Scanning)
- Actively looking while physically interacting. (Searching)
(I use the word “physically” here, but the same conceptual breakdown can apply to non-physical scenarios: For example, the difference between noticing a hostile malware attack, actively scanning a program without loading it into memory, and actively scanning a program while running it. Or unexpectedly noticing that a girl on the far side of the bar is smiling at you vs. specifically scanning a social event in order to identify the Skrull shapeshifter vs. actually joining the party and talking to people to see if you can get the Skrull to slip up.)
These conceptual tiers exist even when the system doesn’t necessarily distinguish between them, so I’ve found it useful, when approaching a new system, to give a little thought to how I want to handle them.
For example, 3rd Edition D&D distinguishes between passive perception (Listen/Spot checks) and active perception with physical interaction (Search checks). You can also have active perception at a distance if you spend an action to take an additional Listen/Spot check. For my house rules, I stated that passive Spot checks can be made against Search DCs at a -20 penalty and that Search checks could be made without physical interaction at a -10 penalty. (Collectively, this creates a zone of overlap between the skills and provides guidance for the previously uncovered zone between the two.)
DIVIDED PERCEPTION
As the example of D&D suggests, the difference between passive perception, scanning, and searching is not the only way perception skills can be divided. They are also often divided by different methods of perception (Listen vs. Spot, for example). In other cases, there may be only one skill, but it may come baked with a preconception about which sense(s) are involved in perception (Call of Cthulhu, for example, only has a Spot Hidden skill).
Regardless, I think it’s important not to get stuck on the idea that sight is the only sense that can be used to notice things. Get sound and scent and touch involved.
Also give some thought to how you can adjudicate situations in which the “wrong” sense is used to perceive something. Can you “hear” a secret door, for example? Maybe if there’s an unusual movement of air or some dim and distant sound echoing from beyond it. Normally you’d expect to detect an invisible person by listening for them, but if you were attentive enough couldn’t you perhaps notice the slight depression of the carpet beneath their feet?
In the case of D&D, I simply applied appropriate penalties for this sort of thing as the situation warranted. I eventually got tired of the divide between Listen and Spot for a number of different reasons (low utility from the division, causes problems with stealthy approaches, chews up skill points, etc.) and conflated them into a single Perception skill. This required figuring out what the effective loss of a particular sense would entail (not being able to see the invisible; not being able to hear in a zone of supernatural silence) and after some experimentation I went with a simple -10 penalty per sense you’re effectively lacking against a particular target.
Pathfinder went even further, conflating Listen, Spot, and Search all into a single Perception skill. But this actually provides something of a cautionary tale, because they failed to clearly establish a 
methodology for passive vs. active Perception tests, and thus rather bungled the result: If you strictly follow the rules as written, you don’t actively search for traps in Pathfinder. Perception tests default to being reactive (i.e. passive) and nothing in the rules for traps overrides that default. This does not appear to have actually been the designer’s intention, however, because, among other things, there’s a class ability which allows the Rogue to reactively notice traps they come within 10 feet of (which would be meaningless if that was, in fact, the default rule for all characters).
The point here is not that there needs to be a mechanical distinction between passive perception and active searching or between hearing and seeing or any other such division. Maybe such distinctions aren’t relevant to you or meaningful within the abstraction of the mechanics in the system you’re using. Or if there is a mechanical distinction, maybe that isn’t reflected in varying difficulty levels (as in my 3rd Edition D&D house rules). D&D 5th Edition, for example, distinguishes between passive perception and active perception (the former using a flat value vs. DC, the latter using a random roll vs. DC), but doesn’t vary the difficulty of the check based on whether you’re standing in the doorway looking into a room or actively tossing the joint.
The point is that these broad conceptual distinctions will arise naturally out of fiction-first declarations of intention and method during play, and because perception-type tasks are so ubiquitous and so frequent, you are going to want to have a very clear procedure for handling them in a way that both maximizes their effect and cleanly keeps the game moving forward.
On the topic of 5e, there the DM is actually supposed to judge the situation and give different DCs or advantage/disadvantage if appropriate for standing in the doorway or tossing the joint. This also extends to which senses apply etc.
You haven’t written much about 5e so I’ll add a couple extra points.
In 5e the distinction between active and passive isn’t really on what it represents in-game (not actively looking versus scanning), but on how it is adjudicated. Passive here just means there is no roll because the DM want to keep things secret, or because there is no point in rolling an active scanning check every round, taking the average result instead. A little confusing, yes.
5e also has the Investigation skill that can possibly be used to cover some of the searching. Thanks for suggesting how scanning and searching can be used to differentiate between player-initiated Perception and Investigation.
You seem to be quite flexible in when to call for checks and how to apply the rules. Yet it looks like you prefer hard and extensive rules, instead of the DM just judging the situation and setting a difficulty by fiat considering all those same factors. Is that a reason for why you have been lukewarm to 5e?
So if a girl is smiling at me from across the room, she must be a Skrull huh?
In the case of 3e, i think the idea of taking 10/20 is intended to come into play for “tossing a room”/ The idea benig you are making the choice to methodically go over the space/take your time.
Mechanical fiddly bits aside, the overall approach you describe makes sense.
For 5e perhaps a model like:
Passive – perception check at disadvantage (lower of 2d20)
Scanning – regular perception check (1d20)
Searching – perception check with advantage (higher of 2d20)
That’s just off the top of my head having read the article but it would use existing game mechanics rather than try to get the group to accept something new.
A while ago I was involved in a thread on ENWorld that turned into a discussion of how to detect traps. One person said that they require a Perception check to notice the trap, an Investigation check to “figure out” the trap, and a thieves’ tools check to disable it. Personally, I think that’s a terrible idea; it creates multiple attribute dependency (Wis, Int, and probably Dex) for rogues who want to be able to deal with traps, and requiring three rolls for every trap increases the chance of failure.
Although the section on traps in the 5e DMG does seem to support the three-step method, I think there’s evidence in both the PHB and the DMG to support the idea that the designer’s intent was to allow *either* skill to be used for trap detection. Both of them refer to finding a hidden object as one of the uses of the skill, and the index of the PHB, under “trap, finding” says “See Wisdom: Perception. See also Inelligence: Investigation.”
I would rule that in 5e, passive perception is, well, Passive Perception, scanning is an active Perception check, and searching is an Investigation check. (There are one or two mentions of Passive Investigation as well, which as I understand it is intended as a way to avoid excessive dice-rolling when methodically searching a large area.) If the PCs encounter, for instance, a chest with a poison needle trap, a successful Investigation check would detect the trap’s presence; however, even if they don’t explicitly state that they’re searching for traps, I might tell a player with a sufficiently high Passive Perception that they notice a small additional hole above the keyhole.
I’d probably be fine with the three step process of that trap process if the DM (a) allows player expertise to trump character expertise and (b) a failed Investigation check doesn’t trigger the trap. IOW, the Investigation check is a completely optional appeal to the mechanics if the players can’t figure out how to follow up on the initial clues. (And, to some extent, so is the Disable check.)
Looking at my 5E books, that appears to be the intention. Although they outline the three step process, they note: “In most cases, a trap’s description is clear enough that you can adjudicate whether a character’s actions locate or foil the trap. As with many other situations, you shouldn’t allow die rolling to override clever play and good planning.” And then elaborate with examples of player expertise trumping character expertise.
I run a Call of Cthuhlu campaign, and I’m sick of how the spot hidden and listen skills work. They’re either extremely necessary to the point were they suck up my players skill points when making characters, or entirely useless because the one player who’s in the position to notice something doesnt have the skill.
So I started to ask them more what they were searching and how, put the emphasis on interaction rather than rolling, and drop in a dice roll if they’re doing something difficult or they need help.
Only problem is, this doesn’t cover pasive perception, I don’t want to straight out tell them that they’re being followed by a creepy car, and I can’t exactly ask them if they’re looking behind them.
I think passive perception dice rolls can be useful, because if they fail, they’ll be nervous about if there’s something they should be worried about. So far I’m having a toss up between getting them to roll for intelligence, or make a new skill adding their spot and listen skills together?
I like to divide it up into noticing, receiving, or transmitting, myself.
Transmitting is interacting to get a reaction, which can be separated out from the observing part. Like a radar transmitter or a sonar system bouncing waves off of the target, which other radar and sonar systems in passive modes can pick up without having to do the interaction themselves. Or a power-armored soldier conducting reconnaissance by fire by shooting their minigun into the brush while a concealed ninja watches the brush for signs of enemy movement. Moving beyond military examples, an old duke might bring their spitfire friend to a meeting with an unknown person so that the spitfire will poke and prod the unknown while the duke takes their measure from their reactions.
This conceptual separation is useful because it emphasizes the possibilities of and opportunities for deflection and concealment. The active sonar and radar systems might be attached to a battleship or tank or flying fortress which can take the counterfire, while the passive systems could be small power-armored soldiers that make their way along the ground guided by the probing of the heavier forces. The spitfire’s aggressive manner draws negative reactions to them, but not necessarily the duke that invited them (or perhaps the duke just told them about the meeting knowing that the spitfire would invite themselves to it…).
Receivers and those who notice can ride off of the interactions of the transmitters, and transmitters may be very good at getting a reaction but very bad at noticing them. This separation can make perception more of a team effort, which can be useful in some circumstances.
I forgot to note that transmitters (or maybe just emitters) can also not be characters at all. A full moon casts a dark forest in light, allowing soldiers to receive the glint of moonlight off of polished steel. The sudden entrance of an ex-husband at a party reveals the complicated emotions of the ex-wife on their face for others to see. Arranging or avoiding such circumstances can be a part of the delicate dance of intelligence and counter-intelligence that can drive games and campaigns centered on such activities, though it can also bog down others for which they are peripheral.