DH had a close childhood friend. They were in their early 20s and friend had come over to hang out (just for the evening not to stay over).
DH was still living with his parents at the time and they were all in the living room chatting when friend randomly gets up and announces "right I'm going for a bath" and just goes off and has a bath in their house.
It's such a small thing but so bizarre. What odd habits have you seen from house guests?