As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
(CAN) AIRCRAFT HAD BEEN REFUELED AND PREPARED FOR DEPARTURE WHEN THE PILOT NOTICED THE LEFT HAND AUXILIARY FUEL CAP MISSING. AIRCRAFT RETURNED TO BASE AND IT WAS DETERMINED THAT THE CAP COULD BE LOCKED AND STILL RELEASE FROM THE TANK. NEW CAPS WERE INSTALLED ON THE AIRCRAFT, P/N 100-381005-1.