Go to the multiplayer map "Airfield". Go to the first side, which is the one where you spawn behind the big building then go out to the field. Instead, go inside the building and examine it. Look around at the pillars and windows, then go upstairs to the left. Look around some more and on the balcony. Notice it is the same building in the Nacht Der Untoten (zombie) mission, except all the entrances are walled up or blocked by an invisible wall.
