It's a thoroughly standard word applied to two things: the Queen, and a small missing object. The Queen goes walkabout whenever she visits anywhere, and you can complain that your pen has gone walkabout.

Now that's odd, I was just about to say a synonym for the missing things is "grown legs". But something that's gone walkabout is probably just misplaced and will reappear at some point, but something that's grown legs is probably gone for good and you have to get another one from the stationery cupboard.