What was found
AIR TURN BACK. AFTER TAKEOFF, MASTER WARNING FIRE CARGO AFT. QRH PROCEDURES COMPLETED. ACFT RETURNED & MAIN DECK CARGO COMPARTMENT PHYSICALLY CHECKED. NO EVIDENCE OF SMOKE OR FIRE. SUSPECT FAULT OCCURRED DUE TO MOISTURE. STATUS MSG, MD CARGO 15,11,10 LP A, MD CGO 4,9,16 LP B. FOUR SMOKE DETECTORS REPLACED & TESTED OK.