Sometimes chams mark an unfamiliar climbing branch (your arm) by rubbing their vent on it. Some people think it is a territorial scent mark or a "I was here" message. They tend to do it when outside their known territory. He could also have a bit of something irritating the vent too. Some chams wipe themselves on branches. Hard to say. Most of my male chams did this once in a while, but I suspect the reasons varied. It was worth a laugh but that's about it.