What was found
(CAN) AFTER TAKE-OFF AND GEAR RETRACTION, 'NLG DOOR OPEN' MESSAGE APPEARED ON ECAM. GEAR CYCLED, BUT MESSAGE REPEATED. AIRCRAFT RETURNED FROM FLIGHT. POST FLIGHT REPORT INDICATED NOSE GEAR DOOR PROX. SENSORS 30GA AND 31 GA FAILED. BOTH SENSORS REPLACED, AND SYSTEM CHECKED SERVICEABLE. AIRCRAFT RELEASED TO SERVICE.