Monsters that are hiding out of the gamespace, is a common problem yes... Because the Battlefields are randomly generated, you can immagine how difficult it is to eliminate that completely.
Compared to the initial release of the game (before patches), the patches has removed a LOT of those hidden monsters.... but a few still remains.
The poison arrows can currently shoot through a couple of places in the walls, so if you are REALLY persistent, you can always take the extra time and kill them off.
IMO the battlefields are long enough as it is, without having to spend the extra couple of hours trying to kill harmless puppies (with bad teeth).