There’s a lot of ambiguity about perception checks and how and when they work. Here’s how I divide it up.
There’s two situations with a trap.
- A player is about to trigger a trap that they don’t know about.
- A player is looking for a trap on a suspect object (i.e door, etc.)
The first is a passive event, however nobody likes to use their passive perception here. The high perception person will feel cheated if their passive perception isn’t high enough to see the trap, and the low perception feels almost as cheated. Sometimes I will offer the choice to roll or use their passive to see a trap before it triggers. However, this is only available to the creature triggering the trap. Maybe you can allow a high passive friend to see the trap from 10 feet away before they trigger the trap, but we’re getting into table cost territory here.
The second is obviously a simple check. However, you get into the problem where somebody makes a low roll and insists on rolling again. This is a table cost situation as well, so I’d advise that you let the “searcher” have the option to use the 3.0 option of “take 20” on a search, but say it takes 2 min.
This one is more straightforward, and I think you should use the RAW on this. In other words, the players only get their passive perception to notice hidden creatures. There’s already a die roll involved (i.e. the hidden monster’s Stealth roll), which you can remind a player of if they get upset about using their passive.
This spans everything from spotting a secret door, to the faint bloodstains on a carpet. This is one that takes special care, since success on the roll can move the story forward, and failure can stall it.
As a result, this is one you as the DM want to succeed more often then not. There are a couple tricks to help make that happen.
First, allow a “room search” roll, and let everybody in the room make it once. This simulates a quick and dirty search of the room for anything. Odds are, somebody will roll well, and you can then reveal some story moving detail. To reward a very good roll+skill bonus combination, you can reveal even more information.
The key is setting the DC realistically, with an occasional bump lower when there’s important story detail.
Example 1: The secret door
You can set the DC high here (20), and there’s a good chance that the door won’t be found. Here is a place you would leave the DC normal, since the players probably have other avenues to explore if the secret door isn’t found.
Example 2: The bloodstain on the floor left by the EBG when he killed the missing prince, and dragged the body away to reanimate.
This is probably best at DC 15 realistically, and it’s important to the story. It is unlikely to be missed with 4-5 people rolling. However, your story can survive without the players having the clue that the prince is dead, but his body is missing.
Example 3: The map to the next adventure on the table of other notes.
This is DC 5 realistically, but you will want to bump that down to 0. Thing is, if everybody in the group rolls a 3, you need to conceal the fact that you are simply handing them the map. This is easily done with a “well, you don’t find much, but you do see the obvious edge of map under the papers on the table.” Then they think their poor rolls missed them a detail, when it truly didn’t.
Metagaming low rolls
In the previous example, there’s a potential problem when the entire group rolls low. The players might metagame their low rolls by saying “we now do a extra search of the room”, often with a cruel grin about how clever they feel.
There’s two ways to handle this. The first is to say “that’s metagaming, please don’t do that, it makes for a better game”. If you have good players, that works 9 times out of 10.
The other way is to have a 15 minute argument about metagaming. I’m gonna go with the first option myself.
I have on one occasion said “I will let you know at the end of the session, as long as you promise not to metagame that knowledge with your character”. In many cases, the player is the one who just “has to know”, and it having a hard time getting over the dreaded metagame wall. Help them over the wall, don’t bash both your heads bloody against it.