This requires a camera!

Any chance of parking the bike within view of a computer you have control over? I'd throw a webcam on it for fun.

When I shared an office with some folks, we loved pulling "in good fun" pranks on each other. To give ourselves a heads up, a couple of us installed a webcam with software that would record when it detected motion. I think such a thing would be helpful here.

If you decide to go the vengeance route, definitely make absolutely sure it's him.