Searching

Adventurers must often look carefully to find concealed objects, such as secret doors, disguised treasures, or crucial clues that will lead them to the next stage of the adventure. They make Perception checks to find such items of interest. Unless the group is under a time constraint, the DM should assume that the best searcher is going to roll a 20 eventually and should therefore use the best possible Perception check result that any member of the group can achieve. (In other words, add 20 to the best Perception check modifier in the party.) The adventurers spend a couple of minutes searching, and the DM tells them what they find.
    A published adventure tells the DM what can be found in a room and how hard it is to find. For a DM creating his or her own adventures, the following examples provide guidelines for setting the Perception DC to find a secret object.

Perception DCExamples
EasyAnything valuable in a chest full of junk; something very small or subtle but otherwise in sight
ModerateA valuable item tucked away in an unlikely place; a secret latch or compartment
HardA secret door

Published in Dungeon Master's Guide, page(s) 41, Rules Compendium, page(s) 174.