Gigi Hadid Goes A.M.I.A. ... Ass Missing in Action! (PHOTO)

Gigi Hadid's outfit at the S.I. Swimsuit Issue party left little to the imagination, except this -- we can't imagine where the hell her butt went!

Put out an A.P.B. because Gigi's much beloved ass was definitely missing at the NYC event. We're gonna chalk this up to really, really bad camera angles ... even though the profile shot isn't all that cheeky either.

Let's just remember better days -- Gigi's ass at last year's Tommy Hilfiger show ...