There is no such thing as a passive perception CHECK in 5e... there is passive perception, but that's a static number, and everything that has a lower to-notice-DC then that number, you just, well... see. So, before anything, the characters should atleast enter some form of active perception/search stance to actually do checks for their surroundings, which should - of course - be hidden rolls.

Last edited by WarBaby2; 21/10/20 04:05 PM.