As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
PILOT REPORTED CARGO DOOR OPENED INFLIGHT AT APPROX 500FT AGL, RETURN TO DEPARTURE AIRPORT FOR UNEVENTFUL LANDING. DOOR WAS INSPECTED PER THE AVTEC MANUAL AND OPS CK NORMAL. AIRCRAFT RETURNED TO SERVICE. TRIP WAS COMPLETED WITH NO FURTHER PROBLEMS.