Someone else did. When the door first opens you can JUST see someone exit the frame through the left window. Appears to be shorter than the two drunken pedos. They appear to be very cognizant of staying out of view as they're already in the side room by the time the door is only 1/4 open.

Edit: At the end of the video the door that the 3rd person would have gone through is mostly shut meaning that after retreating into that room they would have had to close the door more than what it would have been.

Someone else did. When the door first opens you can JUST see someone exit the frame through the left window. Appears to be shorter than the two drunken pedos.