D&D 5E New DM: Lost Mines question and Passive Perception?

[Rabbitbait]

One thing I DON'T like about passive perception is that it gives you a chance of missing something if you are actively looking that you would never miss using passive perception. As such, I have houseruled that when you roll perception, you can choose the better of your rolled perception or your passive perception. I know there are all sorts of reasons to disagree with this, but for me it's just a matter of flavour - why should you be better at looking when you are not trying?

 

[iserith]

One thing I DON'T like about passive perception is that it gives you a chance of missing something if you are actively looking that you would never miss using passive perception. As such, I have houseruled that when you roll perception, you can choose the better of your rolled perception or your passive perception. I know there are all sorts of reasons to disagree with this, but for me it's just a matter of flavour - why should you be better at looking when you are not trying?

This is something of a misconception about how passive checks work in D&D 5e, generally due to the influence of previous editions. You are actively doing something (with an uncertain outcome) when a passive Perception check comes into play - you're just doing it repetitively over time. When you make a regular Wisdom (Perception) check, you're just performing the task one time (and that task's outcome is uncertain).

So if the players have their characters search a chamber thoroughly and commit a fair amount of time to the task, the DM can use the passive Perception score to determine an outcome, if the DM thinks that outcome is uncertain. At the cost of time, the players don't risk rolling below a 10.

If it's just a quick rummage, a regular Wisdom (Perception) check may do (again, if the DM thinks the outcome is uncertain).

If the players have their characters exhaustively search the place, taking 10 times as much time as normal on the task, they may just succeed outright, no roll (see DMG page 237).