In Act 2 Part 4, Grim Discoveries, after you feed Wendigo a military man at the first part of the level, wait until a military man comes through the stalagmite "hallway". Run up to him and make him shoot the stalagmites a few times. Kill or feed him to Wendigo, then return to the stalagmite "hallway". Look at the bullet holes. If you look from a side angle, the bullet hole will seem to be in mid-air.