What was found
(CAN) ENROUTE THE CREW OBSERVED LOW OIL QUANTITY ON THE NR 2 ENGINE. FLUID WAS OBSERVED DEPARTING THE DRAIN MAST. THE ENGINE WAS SHUTDOWN AND THE AIRCRAFT RETURNED TO POINT OF DEPARTURE WITHOUT FURTHER PROBLEM. MAINTENANCE FOUND AN AIR LEAK OCCURING AT THE OIL QUANTITY TRANSMITER ACCESS PANEL ON THE OIL TANK. FOAMING RESULTED DUE TO LOSS OF PNEUMATIC HEAD PRESSURE CAUSING THE SYSTEM TO VENT. THE ACCESS PANEL BOLTS WERE RETORQUED ENGINE OIL REPLENISHED. THE AIRCRAFT WAS GROUND RU AND RETURNED TO SERVICE.