What was found
(CAN) A/C RETURNED FROM FLIGHT DUE TO CABIN PRESSURIZATION. UNEVENTFUL LANDING. ROOT CAUSE - AFTER THE F/T TECH CHECKED AND BOARDED THE A/C, GROUND HANDLING PERSONNEL OPENED THE CARGO DOOR WITHOUT THE KNOWLEDGE OF THE F/T. CORRECTIVE ACTION - EXERCISED AFT CARGO DOOR PRESSURIZATION VALVE, DOOR CLOSED, OPERATION CHECKED AND FOUND OK. ACFT HAS RETURNED TO SERVICE.