Everyone has waved back at someone who was waving to the person behind them. The hand is already up by the time you understand, and there is no way to take a wave back, so on the way down it turns into something else: a stretch, a hand through the hair, a sudden need to check a watch you may not be wearing. Nobody has ever been fooled by this, including the person doing it. It isn’t really a disguise. It is a courtesy to the whole street, a way of saying: you saw nothing, and neither did I.