What was found
ON CLIMB-OUT JUST ABV FL190, MAIN DECK CARGO SMOKE WARNING ANNUNCIATED FOR MID 2. PRIOR TO MAIN DECK WARNING LOOP A MID 2 FAULTED. COMMENCED AIR TURN-BACK TO KGRR, RAN MDC SMOKE PROC IAW QRH. ECAM REQUEST TO NORM ON DESCENT THROUGH APPROX 15000'MSL. NO SMOKE OR FUMES IN COCKPIT AT ANY TIME. NORMAL APPR AND LANDING. MAIN DECK (IDENTIFY MID 2 LOOP A SMOKE DETECTOR(S) - INOP." INSPECTED ALL OF MAIN CARGO DECK FOR SMOKE. NO SMELL OF SMOKE FOUND. REMOVED AND REPLACED MID 2 CARGO SMOKE CONTROL UNIT PER A310-200 MM SUP 26-16-12-4. OPS CK GOOD.