Parts are noted in the description of the video, So this guy only runs directly to where enemy's were nowhere else never looking around to see if there are enemy's sitting anywhere else, its a long video, the most important parts are:
8:11 Runs STRIGHT TO look behind a box, there were no audio Q's. Then Immediately runs to the med-bay area to shoot a guy on the other side of that wall he just looked at. 8:28 Pre-Aiming at random corner, and pre-firing. NO NOISE FROM ENEMY.
19:01 pre-fires at wall, no enemy's in sight but there were 2 enemy's exactly where he was shooting. one jumps right from inside that room, the second at 19:06 comes around left from that long hall for his first death. 49-1
There were a BUNCH more random oddity's in the video, but those two stood out the most.