I think you should go with the camera idea, because it's possible there are two or three people in on it together, and if the boss catches one guy he'll punish him, think the matter is resolved, the other guys will temporarily stop doing it, and then start a while later. Also, what happens if the guy doing it has an important role at the place where you work? If this is the case, your boss may catch the guy, give him a stern rebuke, and then allow him to keep working there, and you'll be none the wiser as to who was doing this. Remember business always comes first for a boss.
I don't know much about technology, but these days you could probably buy a tiny video camera that could be hidden away. Make sure it is well hidden though. Put a jumper up on the area beneath the back windscreen with the camera hidden in it. Arrange the jumper in a mangled position where the camera can record what is going on from out of a cranny (within the jumper). Push it in deep so it isn't visible but is covered by darkness. Rememer, the sun changes position throughout the day: you don't want light shining in the cranny so the camera can be seen.
If you are willing to sacrifice a few dollars you could even hire a private investigator.