What was found
FLIGHT 4012 WAS CLIMBING OUT FROM PIT AND THE CREW RECEIVED A DOOR CHIME AND A RED WARNING PANEL INDICATION OF A OPEN PASSENGER DOOR. THE FIRST OFFICER VISUALLY CHECKED THE DOOR AND DOOR HANDLE FOR SECURITY. THE LIGHT WENT OUT THEN RETURNED. THE CREW REFERENCED THE QUICK REFERENCE HANDBOOK AND CALLED MAINTENANCE CONTROL. THE CREW WAS INSTRUCTED TO RETURN TO PIT AND LANDED WITHOUT FURTHER INCIDENT. MAINTENANCE INSPECTED AND CLEANED THE OVER CENTER PROXI SENSOR TARGET IN ACCORDANCE WITH DORNIER MAINTENANCE MANUAL 52-00-00. OPERATIONAL CHECKS WERE PERFORMED AND THE AIRCRAFT WAS RELEASED FOR SERVICE.