The 63-year-old -- who was born in Mexico City -- had traveled south of the border to handle a property he inherited from his mother who passed away a few years ago.
Friends of Salkind tell us ... members of Ilya's staff last saw the legendary producer at his estate in Mexico on Sunday as he was on his way to "handle some errands and get some dental work done."
However, Salkind has not returned -- and yesterday, his friends filed a missing persons report with Mexican authorities.
Salkind's credited with securing the rights for a live-action Superman movie back in 1974 -- and served as Executive Producer on Superman I, II, III.
UPDATE: Ilya's friends have already launched a website to help track him down.