What was found
WHILE EN-ROUTE, THE CABIN DOOR (OPEN) LIGHT ILLUMINATED. NO DOORS OPENED AND THERE WAS NO LOSS OF CABIN PRESSURE. THE AIRCRAFT LANDED UNEVENTFULLY WITH NO EMERGENCY OR PRECAUTIONARY PROCEDURES TAKEN. INSPECTION BY MAINTENANCE PERSONNEL, DISCOVERED A DOOR HOOK SPRING ON ONE OF THE MAIN ENTRANCE DOOR HOOKS THAT HAD COME LOOSE CAUSING THE HOOK TO MOVE AWAY FROM THE DOOR OPEN SWITCH FAR ENOUGH TO ILLUMINATE THE LIGHT. THE SPRING WAS INSPECTED AND RE-ATTACHED AND THE AIRCRAFT RETURNED TO SERVICE.