I can tell you why it happened Matt
It's because either Excel crashed while he had the file open - or because his computer crashed or was turned off incorrectly.
I've had this happen at a few of our clients before, and unfortunately there was no real "fix"
I even when to the Open Files area (Comuter Management) on the server and closed the files from there...didn't fix it. Crappy thing is this was a mine site, so we couldn't just "restart the server at the end of the night" because they work 24/7
:/