It could be a properly executed loop vent...I'm not sure. Somewhere, there needs to be a vent line going up, but it might be in a different wall.
If there's a partial blockage beneath you, it can build up and will come out the lowest point possible. This might be your toilets or the tub, but there may be multiple lines before they combine and the only thing on the sink line is the one that is partially blocked and that's why it is confined to that sink.
It can get messy in a condo association convincing the others that it is their problem, and not yours. If you pay a management company, sometimes it easier to get them to understand, but it is the condo board you have to convince.