As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
THE PILOT AND MEDICAL CREW SMELLED FUEL SHORTLY AFTER TAKEOFF. THEY IMMEDIATELY RETURNED TO BASE AND FOUND THE FUEL CAP MISSING FROM THE ACFT. THE FUEL CAP WAS NEVER LOCATED, SO THE CAUSE OF WHETHER THE CAP WAS INTALLED INCORRECTLY OR IF THE LOCKING MECHANISM FAILED IS UNKNOWN. THE PROBLEM WAS CORRECTED BY INSTALLING A NEW FUEL CAP.