i'm only guessing here...but what are the cahnces of another router picking up from yours? Since there's so many other apartments and houses around you, would another not be able to pick up a signal?
You can hook up quite a few computers to a router in the home (i forget if they require a special serial number to work) but with some hacking i doubt another computer you don't know of can't pick up your signal.
Or maybe that's the way videotron works
we used to have a router and we never had hacking problems but this was in a house not a building.
good luck finding your thief