I've had guys scratch their crotch all the time... but I actually knew a guy would stroke it thru his paints. His MO was to sit down and put his hand on his thigh. He was hung large, and his cockhead was always in his right pant leg. His hands were large enough that he could take his thumb and run it up and down his cockshaft.
It was pretty subtle, but once you clued into it, you realized that everytime he sat down with his hands free, he'd be massaging his tool. And yeah, it would get larger. And yeah, he did it with everybody.
He used to swim naked, and I know it was a fat UC job... I think he just liked touching it.