THE PLT DEPD SOUTH FROM AN UNATTENDED ISLAND ARPT AT SEA LEVEL & FLEW INTO MARGINAL VFR CONDS WITH MODERATE SNOWFALL. ACQUAINTANCES AT THE ARPT RPRTD THE PLT EXPRESSED AN URGENT NEED TO RETURN TO SNOHOMISH, WA, DESPITE THE WX CONDS. WHEN THE ACFT DID NOT ARRIVE, A SEARCH WAS INITIATED. A USCG HELICOPTER CREW LOCATED THE WRECKAGE BY HOMING ON A WEAK ELT SIGNAL THE NEXT DAY. THE ACFT HAD HIT TREES ON A KNOLL IN MOUNTAINOUS TERRAIN ABOUT 4 MI SSW OF THE DEP ARPT. IMPACTOCCURRED ON A NORTHERLY HEADING AT AN ELEV OF ABOUT 1400' MSL. THE FLAP HANDLE WAS FND IN THE 2ND NOTCH. NO PREIMPACT PART FAILURE OR MALFUNCTION WAS EVIDENT. THE PLT WAS NOT INSTRUMENT RATED. AN AIRLINE EMPLOYEE, WORKING OUTDOORS SOUTH OF THE ARPT, OBSERVED THE ACFT DRG THE INITIAL CLIMB. HE RPRTD THE ACFT APPEARED TO BE OPERATING NORMALLY & ESTIMATED THE CEILING IN THAT AREA WAS ABOUT 1000' AGL WITH 3 MI VIS & MODERATE SNOWFALL. THE ARPT ELEV WAS 25'. ABOUT 18 MI WEST,THE WX WAS IN PART: 400' PARTIAL OBSCURATION, 1300' BROKEN, VIS 2.5 MI WITH SNOW, WIND 270 DEG AT 4 KTS.