FAA INSPECTOR CATCHES A POSSIBLE ACFT MAINT DISCREPANCY DURING RAMP INSPECTION.

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ARRIVED AT GATE LATE DUE TO WX WHILE PERFORMING TURNAROUND DUTIES; A LCL FAA MAINT INSPECTOR CAME TO THE COCKPIT AND TOLD ME I 'NEEDED TO LOOK AT MY AFT LAVATORY.' WHEN ASKED WHY; HE SAID IT WAS LEAKING. SINCE WE HAD BEEN WELL BRIEFED BY OUR COMPANY CONCERNING THE DANGERS OF FROZEN BLUE WATER; I WENT TO INSPECT THE AFT LAVATORY SVC PANEL AND SURROUNDING FUSELAGE AREA. THE FUSELAGE HAD SOME VERY MINOR STREAKING WHICH WAS VERY DISCOLORED; AND EVIDENTLY VERY OLD. THERE WAS SOME MINOR DRIPPING FROM AROUND THE DRAIN VALVE STOPPER; BUT NO EVIDENCE OF BLUE WATER FLOW. I CHKED WITH THE RAMP SUPVR; WHO TOLD ME THE LAVATORY HAD JUST BEEN SVCED; AND THAT WHAT I SAW WAS VERY COMMON FOLLOWING SVCING. I TOLD THE FAA INSPECTOR I SAW NO PROB. HE QUESTIONED MY OBSERVATION; AT WHICH POINT I ASKED IF HE WAS EITHER DEMANDING FURTHER ACTION OR GNDING THE ACFT. HE REPLIED; 'IT'S YOUR CALL.' I THEN TOLD THE RAMP SUPVR TO REDUMP AND RESVC THE LAVATORY; AND WE WOULD THEN INSPECT FOR ANY LEAKING. THIS WAS DONE AND A VISUAL INSPECTION REVEALED NO EVIDENCE OF ANY LEAKING OR DRIPPING. THE FAA INSPECTOR HAD LEFT THE SCENE; AND FINDING NOTHING WRONG; I DECIDED TO CONTINUE. I HAVE SINCE BEEN INFORMED THE STL FAA MAINT WANTS TO FILE AN INVESTIGATION. OUR POI FULLY SUPPORTS MY ACTIONS. AS I MENTIONED EARLIER; OUR COMPANY HAS FULLY INFORMED US ALL ABOUT PROBS WHICH CAN DEVELOP WITH FROZEN BLUE WATER; SO THERE WOULD BE NO QUESTION ABOUT CONTINUING IN THE FACE OF A LEAK. I'M SURPRISED THE STL FAA MAINT INSPECTOR WOULD LEAVE THE SCENE BEFORE THE PROB WAS RESOLVED IF HE REALLY THOUGHT SAFETY WAS INVOLVED.