Was there a closet on dead air? I don't recall one for that specific finale.
Yeah, there is one. Some guy found it while me and Sarevok were playing. It's near the edge of the map in the building. Just go left of the turret and you will find it.
It is pretty cheap, though. There's a reason why the maps are edited for Versus mode. In fact, you can apply the same tricks in every single level when you have to prepare for the horde. Having 4 people sit in a corner while you melee basically makes you unstoppable. No infected class can do anything to you at all. I know there's also a cooldown time for melee if you join certain servers in Versus(it might be all of them I'm not sure), because melee is basically a frontal shield. I'm only really using the closet method to help me get achievements, anyway. Everyone is already picking up on it.