I was exploring on level 4, and hit a Zoo. But instead of there being just one Zoo on the level, there were 2 (separate ones). Upon discovering the first, the second one was also revealed on the map, and the monsters in it were counted towards the "monsters left". I hadn't even explored that section of the map. So probably should either only have one Zoo per level, or have some way to count the Zoos separately if there are more than one around. Not that I'm complaining about getting 2, the XP is great.