I shall: I've got an adventure that says my players could *spot the hidden monsters* by passing a Perception DC of 10. I am stumbled as any adventurer will already have a passive perception of *at least* 10 - so anyone should see them without having to pass an active check (unless they got 9 or less WIS that is)...
Assuming that is a mistake on the adventure's side, what would be a proper Perception DC for hiding a group of rats in a *very dark cellar*, behind barells and such?