I have had several instances where during message board maps, one of the required enemies (orange text that you have to kill to complete the level) is just outside the accessible area. It happens most often in the underground cavern maps (where the enemies are basically WAY below), but I have also seen it a few times in the forest (where I can usually still see and shoot them, though yesterday there was one that I think was up in the trees based on where my missiles were trying to go), and I also just had one on the outside of the wall in the cathedral type place (as if I were going to the vault with the aurora knights). That one was a deomn of flesh and almost pulled me out with him as he fell (As it was I ended up stuck IN the wall and had to portal back and go in the starter entrance).

The good news is that I can't once remember this happening on expedition maps. Even though we aren't required to kill certain enemies, I would be more likely to miss them, but I still should have noticed a place where I carefully approached a red skull on the map and yet couldn't find the enemy.