As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
THE AFT CARGO DOOR LIGHT ILLUMINATED IN FLIGHT. CREW FOLLOWED THE CHECKLIST AND RETURNED TO EWR WITHOUT FURTHER COMPLICATIONS. MX MET A/C AT GATE AND FOUND THE CARGO DOOR HANDLE NOT FULLY CLOSED, POSITIONED HANDLE CORRECTLY AND PERFORMED OPERATIONAL CHECK OF THE DOOR IAW Q400 AMM 52-31-00.