The Australians use the word to explain why someone is missing (is it always a male thing by the way?) from his usual places – “He’s gone walkabout.” In fact he’s gone off somewhere out of contact and on his own. This seems in keeping with our using the word to refer to a missing object, but not when we apply it to the act of the Queen, or whoever, when going around the fringes of a crowd and having a few words here and there. She is certainly not missing, she is there in plain view, just walking about. It's a neat enough way to describe the action though.