What was found
(CAN) AFTER TAKE-OFF AND AS THE AIRCRAFT BEGAN TO PRESSURIZE THE CARGO DOOR WARNING LIGHT ILLUMINATED. THECREW REQUESTED A RETURN TO BASE AND LANDED UNEVENTFULLY. MAINTENANCE INSPECTION OF THE AIRCRAFT DET ERMINED THAT A FALSE WARNING INDICATION OF THE CARGO DOOR WARNING SYSTEM WAS CAUSED BY THE PRESENCE OF MOISTURE IN THE AREA OF THE CARGO DOOR WARNING SWITCHESAND CLICK-CLACK LATCHES. THE SWITCHES AND LATCHES WERE DRIED, CLEANED AND LUBRICATED AND THE DOOR CLOSURE AND INDICATING SYSTEM FUNCTION CHECKED SERVICEABLE.