As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
(CAN) CREW REPORTED THAT PRIOR TO THE LANDING GEAR RETRACTION, THE FUSELAGE DOORS, CARGO DOOR, MASTER WARNING ANNUNCIATED. ACFT CLIMBED TO 4000 FT AND DIFFERENTIAL WAS ZERO. FLIGHT RETURNED TO DEPARTURE AIRPORT WITHOUT INCIDENT. MAINTENANCE FOUND THE AFT CARGO DOOR HANDLE HAD POPPED OUT. HANDLE WAS INSPECTED AND RESET THE ACFT DEPARTED WITHOUT FURTHER INCIDENT.