We've been practicing waving and saying bye bye for a long time now. Sometimes they seem to wave or at least there's a movement of a hand. Sometimes not.
But yesterday as HN was leaving, we were in the living room and I was waving and saying "bye bye" and Grunter, without moving his hands at all, looked up at HN and said "bye bye".
And then he went back to playing while HN and I squealed with joy!