There may be an additional valve somewhere upstairs. This would be desirable to be able to service it without impacting the first floor tenant. Maybe in a utility room, behind a panel, in a closet...hard to say. The other, much more problematic, is that maybe during the winter, the heat was off, the pipe split, and it's running in the wall(s) somewhere. With everything off, check the water meter (assuming you have one) and see if it is registering any water flow. If it is, shut off the main, then go searching!