The university likely has rules regarding this, generally, that they shouldn't, but YMMV. Seems to me, the "accidental" pick-up seems a little silly. I suppose its possible in a large lecture hall of students, to not see every face. But once you know someone well, it should become obvious to the people involved, and they could mutually agree to abide by the rules. Obviously, if nosy people in administration watch the professors like hawks, then they could be caught unawares. But there's probably a review process for infractions.