It seems that Ant-Man will begin filming tomorrow, Monday, August 18.
That's based on fan-shot images circulating that say a film called Bigfoot will be filming in San Francisco on Tuesday after a day of prep work. Bleeding Cool noted that the flier identifies the production company as Pym Particles Productions, an obvious reference to Michael Douglas's character in Ant-Man.
Most major motion pictures shoot under some kind of alias to minimize onlookers. Iron Man 3 was Caged Heat and later just Caged while Batman V Superman: Dawn of Justice is Sage and Milo.
That's just the beginning, though; according to numerous reports (first Latino Review, but it's been picked up elsewhere since as "sources" rather than relying on the original report), one of the first scenes shot will be a flashback sequence that will feature Dominic Cooper (Howard Stark), Hayley Atwell (Peggy Carter) and Toby Jones (Arnim Zola) in a scene that would tie a young Hank Pym and a young Alexander Pierce to the roots of S.H.I.E.L.D. and, presumably, to Marvel's Agent Carter.
No word on who would play the younger versions of Pym and Pierce, who are played by Michael Douglas and Robert Redford, respectively, in the Marvel Cinematic Universe.
Not all versions of the rumors include Pierce, although everyone is pretty consistent on the rest of the players involved. Flashback sequences featuring at least Peggy Carter have been rumored for Avengers: Age of Ultron as well.
Ant-Man will be directed by Peyton Reed. It stars Douglas alongside Paul Rudd, Evangeline Lilly and Corey Stoll.