Today while we were waiting to board the subway, Mark had quite the jolting experience. A lady who appeared to be in her 50s, sidled up behind him and firmly grabbed his behind. With a shocked expression, he turned around to see who or what had violated him. He saw the lady move on to the person standing next to him and give them the same treatment.
We have not figured out what sort of custom this is.... but in the meantime, it's definitely highly entertaining (more for me than for poor Mark).