If a wandering mob is spawned from a lair and is destroyed it thinks you have destroyed the guardian and thus shows a green treasure chest above the lair. I thought that this was a simple enough bug, but in actual game terms it is not.

The tag on some lairs is destroy if guardian is destroyed, but this means that the lair is no longer a threat the moment you kill a weak guardian travelling about. For example a brood lair spawns a shrill with brood archer, I kill him and now the lair is no longer there leaving me in relative safety since the lair will no longer spawn new wandering mobs and the guardian mobs will not move.