I think they react to each other the same as they react to human driven cars, ie there’s no programming of “that cars another Waymo so treat it differently” it seems like one of them sees “that car is about to back in to me” so it executes “friendly honk so they don’t hit me”. When maybe there should be a “it’s another robot, so just trust them” check in the logic, especially when it’s the middle of the night.