How smart is the roommate? You don't need an anti-theft GPS if he can be convinced it has one.
Wait til he's home, slip out, have a friend call the apartment posing as cop. Leave message that they have a signal on the anti-theft GPS and they'll be going to get it and pick up the perp within the next hour. If he has it, he'll try to dump it or move it ahead of the arrest. Follow him, hilarity ensues.
Many years ago my brother's wallet fell out of his pants when he was hanging upside-down from a tree. He didn't notice or see it because it was after dark, so he went home and only realized it was missing the next morning. It was missing the money when he found it. The driver's license was removed and tossed on the ground, so the thief knew who it belonged to.
Our friend, who we trusted, had skipped school and was home, but he said that a certain person from the neighborhood had been banging on his door before the bus came... again (he did this pretty often). He ignored it at the time.
Because this annoying guy showed up there predictably, we made it appear to be a set-up that we video-recorded. We simply approached him with a blank tape in hand and pretended that we had already watched it and already knew that it was him. We convinced him by saying "...and don't even pretend that you didn't know who it belonged to! You looked right at that driver's license before tossing it away."
He returned $17 of the $20 on the spot (he had already spent some).