As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
DURING CRUISE FLIGHT, LANDING GEAR DOOR OPEN LIGHT ILLUMINATED. LIGHT COULD NOT BE EXTINGUISHED USING COCKPIT OPERATING PROCEDURES. FLIGHT RETURNED TO DTW AND LANDED WITHOUT INCIDENT. MAINTENANCE FOUND THE LEFT MAIN GEAR DOOR POSITION SWITCH BAD. REPLACED SWITCH. OPERATIONAL CHECK NORMAL. AIRCRAFT RETURNED TO SERVICE.