DETAILED METHOD to get the boss to spawn:
1. Do not scan from above or before entering the area (I did this and I spent 17 minutes trying to find what I had missed....)
2. Enter FROM THE FRONT like I do here.
3. Wait for guy to die but don't scan him.
4. Scan clockwise from the left where you enter. So stinky guy that Johnny comments on, then puddles, then sheet.
5. Boss will now spawn.