PLT OF C172 INAPPROPRIATELY CHANGED XPONDER CODE TO VFR WHILE WITHIN DC ADIZ AFTER RECEIVING 'RADAR SVC TERMINATED' XMISSION FROM PCT CTLR.

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PRIOR TO DEPARTING HPN; I RECEIVED BOTH A COMMERCIAL FLT PLANNING PROVIDER AND AN FSS BRIEFING. I READ THE ADIZ NOTAM ONLINE; AND THEN CLARIFIED MTN'S STATUS WITH THE FSS BRIEFER. I FILED AN IFR FLT PLAN AND MAINTAINED 2-WAY COM AND A DISCRETE SQUAWK FOR THE ENTIRE 2 HR FLT. I WAS SQUAWKING XXX ABOUT 3 MI N OF MTN. AT THIS TIME; POTOMAC APCH SAID; 'ACFT X RADAR SVC TERMINATED. CONTACT THE TWR ON 121.30.' SINCE I WAS ON AN IFR FLT PLAN; AND HAD BEEN CONTINUOUSLY SQUAWKING THE CODE; I INFERRED THAT MY FLT PLAN WAS BEING CANCELLED AND THAT THE TWR HAD NO RADAR. THEREFORE; I THEN INADVERTENTLY PRESSED THE '1200' BUTTON ON MY XPONDER. AS SOON AS I CONTACTED THE TWR AND ENTERED THE PATTERN; THEY ADVISED WITH URGENCY THAT I SHOULD RESUME SQUAWKING THE CODE. I DID SO IMMEDIATELY. LESS THAN 2 MINS TRANSPIRED. WHILE TAXIING; I WAS ADVISED TO CONTACT POTOMAC TRACON BY PHONE. TRACON ASKED IF I UNDERSTOOD THE ADIZ PROCS; WHICH OF COURSE I DID; INCLUDING MAINTAINING THE DISCRETE CODE. I ADMITTED IMMEDIATELY THAT I UNDERSTOOD WHAT I DID; AND EXPLAINED THAT I HAD BEEN CONFUSED BY HIS ADVISORY THAT RADAR SVCS WERE BEING TERMINATED ALTHOUGH I HAD NOT CANCELLED MY IFR FLT PLAN. FURTHER; THE 1200 CODE HAD BEEN MOMENTARY. HE ADVISED THAT HE UNDERSTOOD; BUT WOULD NEED TO FILE A PRELIMINARY AIRMEN'S DEV RPT. I TRIED MY BEST TO FOLLOW PROC. I UNDERSTAND THE MISTAKE I MADE; DESPITE MY BEST EFFORT. AS I WAS DEPARTING MTN; I OVERHEARD TRACON ADDRESSING AN INBOUND PLT IN THE SAME CIRCUMSTANCES. THIS TIME TRACON ADVISED THE PLT TO CONTINUE SQUAWKING THE CODE. THIS IS AN IDEAL WAY TO ELIMINATE CONFUSION AND PREVENT MINOR INFRACTIONS FROM WELL-INTENDED PLTS.