As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
AFTER TAKEOFF, THE DOOR WARNING LIGHT ILLUMINATED. PILOT RETURNED TO DEPARTURE. NO EMERGENCY DECLARED. LANDED WITHOUT FURTHER INCIDENT. PILOT DISCOVERED DOOR PIN ON LATCH HAD COME LOOSE. RECLOSED DOOR, LIGHT WENT OUT AND DID NOT COME BACK ON.