It will be the homeless lot taking a dump. As there are no shitters to speak of, the floor tends to be used. You do have to watch your step.
Do you ever run into the people who are living in these places? I have read dozens of these adventures and can only recall you running into impromptu security guards and nosy neighbours.
It's very rare, they tend to try it once and find it too creepy to sleep and leave all their stuff behind. If there was a market for used decaying sleeping bags/ mattresses/ filthy blankets I would be very rich.
Better to be rich like you not having to harvest them and just make the huge $$ posting about it on Hive!