This has happened to me about 3 times so far in recent games. I will clear a monster zoo and except for one monster that I can never ever find. I'll start clearing all adjacent rooms as well, and so on, until eventually I will have strayed too far away from the zoo and lose out on the reward. I don't know if the count is incorrect, if a creature spawned inside a wall, or what (I've tried using AoEs at various walls and so far no luck). Ack nm lol. Found the critter 3 rooms away this time back in the direction I came from (he must have fled from my scary awesomeness). At least I know what's going on now.