My brother and I were playing and we found 4 Denizen models strewn around, all outside the maps bounds. Shooting them did nothing until we managed to press a doorbell looking button just to the left of the Power Switch. Once that button was pressed, the Denizens would disappear when we shot them. We shot all four, expecting something to happen, but nothing did. Is that all there is to it? Or is there more we dont know yet?

EDIT: my brother and I found 2 more right after I uploaded this. And yes, just like stated in previous comments, all shooting them does is reward you with a Max Ammo. Fun Easter Egg!