Inspired by the Cloak of Elvenkind thread, I've finally decided to address my biggest source of confusion in 5e. Almost seven years in, and I still get occasionally confused about when Passive Perception is used in lieu of normal Perception. It seems to me that it would be when walking by secret doors, noticing traps or ambushes—sort of like the elf's ability to notice secret doors in AD&D, but that begs the question of when does "active" Perception get used and why?) Then there's Investigation. When does it come into play instead of using Perception? When I search a desk, am I using Perception or Investigation? Lastly, why are the rules for these things so virtually non-existent?
So, okay comunity, what are your thoughts on the matter? How have you parsed these things? And where upon the rules do you base your interpretation upon?
A passive check is used when the DM wants to resolve a task that is being performed
repeatedly and, like all checks, when those tasks have an uncertain outcome
and a meaningful consequence for failure. (A DM can also use passive checks to make "secret rolls" but let's set that aside for now. Personally I think that's an unnecessary use.) Thus, a passive Wisdom (Perception) check is used when trying to determine if a character can spot, hear, or otherwise detect the presence of something while engaged in that task repeatedly. But, again, only when this has an uncertain outcome and a meaningful consequence for failure. If a character engages in a task but
not repeatedly and there is a meaningful consequence for failure, then that's just a regular ability check.
So the question is
when is the character performing this task repeatedly? Normally, it's during
combat and
travel. The rules say that most creatures stay alert for signs of danger all around in combat. So unless the player has described the character as
not doing that or circumstances have aligned such that this is true, the character's passive Wisdom (Perception) check applies. Same goes for travel, which can be measured in feet or miles across minutes or hours. Don't think of this as just cross country journeys or the like - it could easily be movement through a dungeon or other adventure location. A character that is not staying alert for signs of danger, perhaps because by turning his or her attention to other tasks like navigating, drawing a map, tracking, foraging, or similarly distracting activities (as determined by the DM), does not have an uncertain outcome as to noticing a threat - they fail and run afoul of traps or ambushes if they arise. Thus, there is no need for a passive check here. Uncertainty as to outcome is a prerequisite for calling for any ability check.
Notably, a character that is staying alert can also notice secret doors. And this is where Intelligence (Investigation) checks might come in after the secret door is found. A DM can say that opening the mechanism isn't obvious and that figuring it out (depending on the character's approach) might call for Intelligence (Investigation) check. Obviously, if the character pulls all the books on the shelf, for example, and one of them happens to be the lever for the secret door, then no check is required. But this might take more time than deducing the
correct book to pull and pulling just that one. Personally, I separate searching for secret doors as a task on par with navigating, drawing a map, tracking, foraging, meaning that you're not alert for danger if you're doing this while moving about the adventure location. This is because in my games secret doors are excellent finds, concealing treasure, short cuts, or safe places to rest. I want that to come at a cost of possibly traps and ambushes.
Intelligence (Investigation) may also come up when the characters are interacting with a trap. Signs of a trap may be apparent in the environment and deducing from the clues that a trap is present might carry with it an uncertain outcome and a meaningful consequence for failure, so a check is appropriate. After finding the trap, figuring out how to disarm it may also call for such a check, effectively creating a way forward for the rogue to do his or her thing or disarming it some other way (or perhaps setting it off safely or avoiding it altogether). If, however, the clues to a trap are hidden objects, then we're back to resolving finding them by way of a Wisdom (Perception) check, again, when there's an uncertain outcome and a meaningful consequence for failure. With clues in hand, the work of deducing what those clues mean can begin.
The rules for these things are there if you look for them in the PHB and DMG. They are often in different parts of the books - Chapters 7 and 8 of the PHB, Chapter 5 of the DMG, plus of course the general rules for adjudication on DMG, page 237 (the holiest page of the DMG). By way of these rules, it is very likely that a given game sees much more use of passive Wisdom (Perception) checks than Intelligence (Investigation) checks. I see nothing wrong with that. The rules do not suggest in any way (that I know) that these should have parity. But certainly if you are including secret doors and traps in your game and are requiring some amount of deduction be undertaken to engage with them, then you will see Intelligence (Investigation) checks come up pretty frequently. A lot of DMs in my experience don't include these elements in their game sadly and so Intelligence (Investigation) checks don't appear much.