There is an objective in "Outpost" about an S.O.S. message from "Defense". Enter the first Nazi controlled building, where a Demon snatches a fleeing Nazi. Fight through the area until reaching the upper level of that building. Once there you will find a hallway with three tripwire mines and two Demons. Get past the mines and check the right room at the end of the corridor to find a ramp that gives access to the rooftop. You will find a corpse with a Kalashnikov on the rooftop. The communications apparatus for the achievement is found in the corner of the rooftop (Video).