AN SMA HAD AN NMAC WITH AN UNKNOWN ACFT DIRECTLY OVER A MIL ARPT.

Narrative

I WAS FLYING VFR FROM SQL TO SJC. I WAS HANDED OFF TO MOFFETT TWR BY PAO TWR -- REQUESTED VFR TRANSITION THROUGH NUQ ATA ENRTE SJC. NUQ ASKED THAT I MAINTAIN VFR AT OR BELOW 1500 FT. DIRECTLY OVER MOFFETT; I SUDDENLY SAW ANOTHER ACFT JUST BELOW THE END OF MY R WING; COMING DIRECTLY AT ME VERY FAST. IT CAME INTO MY VIEW AND PASSED A FEW FT FROM THE END OF MY R WING IN LESS THAN A SECOND. I THOUGHT WHEN I FIRST SAW IT; THAT IT WOULD HIT ME. IT CAME SO CLOSE THAT I DID NOT SEE THE ENTIRE ACFT. IT WAS A GREY TURBOPROP WITH WINGTIP TANKS. IT WAS IN A HARD L TURN -- I CAN'T SAY IF IT WAS TRYING TO MISS ME OR JUST IN A CLBING TURN. I RPTED THE MISS IMMEDIATELY TO NUQ TWR. THEY REPLIED INSTANTLY THAT THEIR 'RADAR' HAD SHOWN A SEPARATION OF MORE THAN 300 FT. THEY SAID THE ACFT WAS DOING A GAR. SINCE I DID NOT BELIEVE THEIR INSTANT EVALUATION OF THE MISS DISTANCE; I DID NOT PRESS THE POINT AND CONTINUED ON TO SJC WHERE I LANDED. I SUSPECT THAT THE OTHER ACFT WAS ON UHF OR WAS DOING A GND CTLED APCH AND MISSED APCH; OR HAD DONE AN EXTREME PULL-UP -- POSSIBLY TO THE SURPRISE OF THE TWR PERSONNEL -- AND HAD NARROWLY MISSED ME BEFORE THE TWR HAD A CHANCE TO WARN ME. I HAD NOT BEEN ADVISED BY NUQ TWR OF OTHER TFC AND HAD SEEN NONE ALTHOUGH I REMEMBER THAT THERE WAS ONE OTHER XMISSION ON THE FREQ THAT WAS UNINTELLIGIBLE TO ME.