A: "Clean it away and (lightly) tape a white printer paper over one angle of the corner. Do the spots reappear on the paper or on the wall underneath it?

If it appears on the paper but not on the wall, it originates from the airflow. If it is growing from the wall it should first be visible on the wall underneath the paper, then possibly penetrate the office paper.

If it really is coming through the wall and reappearing in a few hours you most likely have a problem behind it. Living things like bacteria or mold don’t grow that fast in room temperature/humidity if cleaned well. But might be a reservoir of something on the floor above that is seeping through."