Maybe it's the dog of one of the neighbours, jk. I'm not trying to justify the act cause it's gross and unacceptable.

We had a similar situation for a couple of weeks with a drunk neighbour who used to piss in the elevator during the night hours. We talked with the building's administrator, with his sister and called the elevator's operator to find a solution and keep the door closed if not used by the residents. This is not a common situation tho and I don't know if you are in an AirBnb or living there.

If the people around you will not collaborate, try to find that MF, call the police /or a shame him with posters in every corner of the building and neighbourhood with posters. Hope this won't be a repetitive situation.