Ah, the exit to that level is in a weird spot. All you need to do is touch a big terminal in one of the rooms in the floor with all the shotgun dudes, and then make your way to that big cafeteria room with the LMG. When you're there, the exit should be a 2nd set of stairs that's to the left of the kitchen, directly perpendicular to the stairs you took to beat the 2nd half of the level. hope this helps!