Squatting? Fix it up, start using it, if the owner doesn't start complaining in a year, it's legally yours. It's generally used for derelict buildings, but in this case it applies. Oh, and you can't make a big secret about it, or it's considered theft instead. But if you're the only one maintaining and using it, it technically becomes yours after a time, assuming you're in the US (you are, right?), and you get to use it in the meantime.
Legally sound, though it's not a written law. Do a quick check with the state laws, first, since a few banned it recently (I think), but it doesn't sound like the dude's using it or gonna miss it at any rate, so even if you're in one of those states...