What was found
ON TAKEOFF, F/A HEARD A POP BY REAR SERVICE DOOR. CLIMBING THROUGH FL 180 GOT A FUSELAGE DOOR MASTER WARNING. ACFT WAS PRESSURIZING NORMALLY, RAN CHECK LIST, CALLED F/A AND THEY REPORTED SERVICE DOOR HANDLE DIDN`T APPEAR LOCKED, SECURED. DIVERTED, DESCENDING THROUGH 12,000 F/A CALLED AND REPORTED ANOTHER POP AND THAT QUITE A BIT OF FROST WAS FORMING ON THE CARPET BY SERVICE DOOR. LANDING UNEVENTFUL. FOUND INTERNAL DOOR HANDLE IMPROPERLY SERCURED C/W IAW MM TASK 52-41-00-710-801. NO DEFECTS NOTED. RETURNED TO SERVICE. NO PARTS REPLACED.