Only 2.7nm from Cooper to Dead Chest - probably took less than 2 hrs to drift there with a 10~15 kt breeze and the usual E->W current. Charterers were probably still waiting for their dinner main course by that point.

Can BoatyBall confirm that their mooring and pennant did not fail?

If the mooring didn't fail, I've got to believe the charterer only ran one mooring line from the bow to the pennant eye and back, and this line was either sawed through by the pennant eye as the boat swung or the charterer tied a very lousy cleat hitch.

If they had run the recommended two independent mooring lines from each bow to the pennant eye, there is minimal chafe at the pennant eye so it would have taken two lousy cleat hitches to both fail. I'm sure Sunsail confirmed the configuration of the mooring line(s) when the boat was discovered.