This might seems like a silly question to ask, but I can't really figure out the answer. The title pretty much says it all.

Let's say you have an open source music player, along comes someone, copies it, adds features, modifies the interface, etc and sells it. Nobody would find out. So how does it work?

Related: I'm working in some projects myself to make me more employable, so employers can take a look at my code but with some of them I don't feel like uploading them to an online repository, ie sourceforge, and make them visible for the general public.