San Antonio Spurs star Manu Ginobili wants answers -- because dude spotted an unidentified flying object in L.A. earlier this month ... and he's desperately trying to figure out what the hell it was.

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Manu eyeballed the UFO when he was in town to play the Clippers on December 1 -- and he has since launched a serious investigation into the matter ... which involved a lot of Googling.

Manu posted his findings on his Facebook page yesterday, saying the best answer he could come up with was "a kind of spacecraft which is called X-37B and was supposed to land in the area between December 3 and 6. According to various media, it landed on Dec 3."

Manu points out that he spotted the craft on December 1 ... not December 3 ... so THE MYSTERY REMAINS.