FLT CREW OF WW24 LANDS AT GAI IN THE DC ADIZ SQUAWKING VFR CODE 1200. NEITHER PLT RECALLS CHANGING FROM DISCRETE CODE.

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ON JAN/WED/04; I RECEIVED A CALL FROM AN FAA INSPECTOR WHO INQUIRED INTO OUR DEC/THU/03 FLT WHICH WAS RPTED TO HAVE SWITCHED FROM ITS ASSIGNED IFR XPONDER CODE TO A VFR 1200 CODE PRIOR TO LNDG AT MONTGOMERY COUNTY AIRPARK (GAI) WHICH UNDERLIES THE WASHINGTON; DC; CLASS B AIRSPACE. APCHING WITHIN APPROX 10 NM OF GAI AND WITH THE ARPT IN SIGHT; WE RECEIVED VISUAL APCH CLRNC; CANCELED THE IFR FLT PLAN; SWITCHED TO UNICOM (123.07 MHZ); AND COMPLETED AN UNEVENTFUL LNDG. I WAS UNAWARE THAT THERE WAS ANY ATC PROB WITH THE FLT UNTIL JAN/WED/04. I AM DUMBFOUNDED AND FLABBERGASTED AS TO HOW THE XPONDER CODE COULD HAVE BEEN CHANGED TO 1200. THE PIC AND I ARE BOTH VETERAN PLTS WITH VAST PLT EXPERIENCE IN GENERAL AND MUCH MORE SPECIFIC EXPERIENCE OPERATING AT GAI ARPT; ESPECIALLY SINCE ENHANCED ATC SECURITY PROCS HAVE BEEN IN EFFECT. WE BOTH KNOW THAT THE ASSIGNED IFR XPONDER CODE IS TO BE MAINTAINED UNTIL AFTER LNDG AT GAI. AS WE APCHED GAI; I DID NOT SEE THE PIC CHANGE THE XPONDER CODE NOR WOULD I HAVE CHANGED THE CODE. SINCE I AND THE PIC ARE CERTAIN THAT NEITHER OF US CHANGED THE CODE INTENTIONALLY; THE ONLY OTHER POSSIBLE CAUSES ARE EITHER AN ANOMALOUS CAUSE OR SOME OTHER UNKNOWN FREAK ELECTRONIC/MECHANICAL FAULT. THE FLT TERMINATED SUCCESSFULLY AND SAFELY. I BELIEVE THAT NEITHER PLT DID ANYTHING WRONG AND THAT NO HUMAN FACTORS WERE INVOLVED.