The man or woman who was already in a quiet public restroom taking a dump, and who stops taking a dump when you enter the restroom or sit down beside them, presumably because they don't want you to hear them taking said dump. They usually wait until you leave to resume their business.
I entered the men's room and sat down for a shit in an available stall. The room was empty, except for one silent partner in the stall next to me. When I was finished, I washed my hands and left, but as the door closed behind me, I heard the whalesong-like sounds of my silent partner resuming his bowel movement.