What was found
PILOT AND CREW DETECTED A BURNING ODOR IN CABIN. RETURNED TO BASE AND LANDED WITHOUT INCIDENT. INSPECTED THE ENGINE BAY, MAIN GEARBOX AREA, UNDER CABIN FLOOR, BEHIND INSTRUMENT PANEL, IN BOTH SIDE CARGO BAYS, INSIDE THE AFT BAGGAGE BAY. NO DEFECTS NOTED. GROUND RAN ACFT IN NORMAL OPERATING MODE WITH AIR-CONDITIONER ON, NO BURNING ODORS NOTED. ACFT RETURNED TO SERVICE. SOURCE OF ODOR COULD NOT BE DUPLICATED.