Obviously the cheaters you would catch while standing over them are the cheaters who would cheat when you're standing over them. Selection bias.

I can tell you with certainty that some cheaters are skittish. I am certain of that because I once wrote a bunch of trig identities on the side of my leg. I was very nervous about it, and only did it because I knew from previous tests in that course that I could get away with it. If the teacher had been walking around during that test, my pant leg would have stayed down.