You know where an aircraft joining overhead is when the pilot reports overhead - it is in the overhead!
In ideal circumstances, maybe. On the other hand, over the years I've seen pilots report "Overhead" an airfield in completely the wrong place and sometimes without first making any other calls when joining. Only a few weeks back I called "overhead" only to hear another pilot, after being quizzed by ATC, also call "overhead" and at the same altitude as I was. He didn't have his transponder selected on, either.
That caused a few anxious moments on our part, but after some frantic head swivelling, I spotted him a long way from the field in our four o'clock. He can't have even been inside the ATZ when he made his "overhead" call and had probably misidentified the field altogether.