PLT IN SMA AT END OF RWY PREPARING FOR TKOF RUN OBSERVED SMA FLY BY AT LOW ALT. OBVIOUS GAR.

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I WAS ABOUT TO MAKE A FLT TO BWI FROM W29 (A NON-CTLED ARPT); WHICH IS A SHORT TRIP I HAVE MADE MANY; MANY TIMES. I WENT THROUGH MY PREFLT CHKLIST IN HABITUAL MANNER; ANNOUNCED MY INTENTIONS ON UNICOM; AND TAXIED INTO POS ON THE RWY. I SAT THERE BRIEFLY MAKING MY USUAL SECOND CHK OF CERTAIN ITEMS ON THE PANEL; ETC. JUST AS THIS WAS COMPLETED; AN SMA FLEW BY ON THE R AT LOW LEVEL IN AN APPARENT GAR. I ASSUMED; WITH ASTONISHMENT; THAT I MUST HAVE TAXIED ONTO THE RWY WHILE HE WAS ON FINAL APCH. I HAD HEARD NO RADIO ANNOUNCEMENT AND HAD NOT OBSERVED HIM WHEN I MADE MY SCAN OF THE APCH BEFORE TAXIING ONTO THE RWY. BUT I NOW REALIZE THAT A DELAY BTWN THAT SCAN AND MY TAXIING ONTO THE RWY MAY HAVE BEEN ONE OF THE CAUSES OF THIS HAZARDOUS OCCURRENCE. IT IS MY HABIT TO HAVE THE CHART OUT EVEN FOR THIS FAMILIAR SHORT FLT; AND I REALIZED JUST AS I WAS MOVING THAT IT WAS STILL IN MY FLT BAG ON THE R SEAT. I STOPPED TO GET IT OUT; WHICH INVOLVED UNSTRAPPING THE BAG; OPENING IT; LOOKING FOR THE CHART; OPENING THE CHART; CLOSING THE BAG; AND STRAPPING IT DOWN AGAIN. I THEN MOVED ONTO THE RWY WITHOUT SCANNING THE APCH AGAIN; PRESUMABLY LULLED BY THE SEEMINGLY INSIGNIFICANT TIME AND NOT HEARING ANY RADIO ANNOUNCEMENTS. I AM STILL MYSTIFIED BY WHERE THE SMA COULD HAVE COME FROM IN WHAT STILL SEEMS TO ME TO HAVE BEEN A VERY SHORT TIME. I HAD CHKED BOTH RADIOS AS USUAL (WITH BWI ATIS) BEFORE TAXIING FROM MY TIE DOWN AND THEN PUT UNICOM IN #1 AND BWI APCH IN #2. AFTER DEP FROM W29; I DID NOT USE #1 AGAIN UNTIL A SUBSEQUENT FLT THAT DAY; AT WHICH TIME IT WAS OPERATING NORMALLY. HOWEVER; I MUST LEAVE OPEN THE POSSIBILITY THAT ITS VOLUME WAS TURNED DOWN AT THE TIME OF THE INCIDENT; BUT I HAVE NO EVIDENCE OF THIS OR REMEMBRANCE OF WHY THIS WOULD HAVE BEEN THE CASE. THERE IS OF COURSE THE POSSIBILITY THAT THE SMA WAS NOT TRANSMITTING HIS INTENTIONS. I HAVE RECOGNIZED ANEW FROM THIS EVENT THE IMPORTANCE OF MAKING THE SCAN OF THE APCH THE ABSOLUTELY LAST THING TO DO BEFORE TAXIING ONTO THE RWY; REGARDLESS OF HOW INSIGNIFICANT A DELAY MAY SEEM. I HAVE ALSO ADDED A SPECIFIC ITEM TO MY FINAL CHKLIST REQUIRING A VISUAL CHK OF THE RADIO VOLUME CTLS.