C182 PLT FOUND THE RWY AT 74S CLOSED; SO HE LANDED ON THE PARALLEL TXWY. HE THEN TOOK OFF FROM THE TXWY.

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FIRST ERROR: FAILURE TO LEARN 'ALL AVAILABLE INFO' ABOUT DEST ARPT. SECOND: JUDGEMENT CALL; LANDED ON TXWY (PARALLEL TO RWY 18/36) AFTER ARRIVING AND FINDING RWY CLOSED (YELLOW X'S). WX WAS GOOD (BY WEBSITE) FOR VFR BFI-74S. GOT ABBREVIATED WX BRIEFING FROM SEA AFSS; DID NOT REQUEST NOTAMS. BRIEFER KNEW MY DEST; SAID NOTHING. ALSO; HAD VFR FLT FOLLOWING AND NONE OF 3 CTLRS (SEA APCH; ZSE; NUW APCH) INFORMED ME OF CLOSURE. SAW X'S FOR CLOSED RWY AFTER CANCELING ADVISORIES. CALLED UNICOM FOR ARPT ADVISORY; NO RESPONSE; BUT AN ACFT ON GND INDICATED HE WAS TAXIING AND HAD EITHER LANDED OR WAS ABOUT TO DEPART FROM THE TXWY. THEREFORE; I DEDUCED TXWY WAS AVAILABLE AND I LANDED WITHOUT INCIDENT. DEPARTED SAME WAY 10 MINS LATER. LATER I PHONED FSS AND LEARNED THE (ONLY) RWY (18/36) IS NOTAMED AS CLOSED; BUT THE ARPT IS OPEN. I PHONED THE ARPT MGR -- NO ANSWER -- I STILL DON'T KNOW WHETHER THE ARPT HAS FORMALLY DESIGNATED THE TXWY AS AVAILABLE AND FSS HAS NO INFO. HOWEVER; THE RWY CLOSURE BUT ARPT OPEN SUGGESTS TXWY IS OK. NEVERTHELESS; IT WAS POOR JUDGEMENT ON MY PART TO LAND AND DEPART WITHOUT KNOWING FOR SURE. (THE ENTIRE TXWY WAS VISIBLY CLR AND THERE ARE NO 'HIDDEN' OR DIFFICULT-TO-SEE ENTRANCES TO IT). BOTH FAILURE TO LEARN ARPT STATUS AND THE LNDG JUDGEMENT CALL WERE MY RESPONSIBILITY. I'LL SURE ASK FOR NOTAMS PERTINENT TO MY DEST IN THE FUTURE. AND NEXT TIME I FIND AN ONLY RWY CLOSED; I WILL LAND ELSEWHERE AND CALL BEFORE LNDG. HOWEVER; I THINK A SMALL AMOUNT OF RESPONSIBILITY LIES WITH FSS AND ATC -- IT'S SURPRISING NOBODY SAID ANYTHING ABOUT THE RWY CLOSURE; EVEN IF I DIDN'T ASK. THERE WERE LOTS OF OPPORTUNITIES!