Nowadays it's pretty rare for high-profile talent to make a movie without news leaking out. Ethan Hawke, Julie Delpy and director Richard Linklater pulled it off when they made Before Midnight together and Linklater did it over a period of 12 years with Boyhood, which featured Hawke and Patricia Arquette.
More recently, we've seen two movies that were produced "in disguise." Producer J.J. Abrams' 10 Cloverfield Lane and director Adam Wingard's Blair Witch were made under different titles with no initially reported connections to their respective originals.
Lowery talked about the project soon after he began working on it, telling Filmmaker Magazine: "I wanted to make something small and tiny and handmade. I don’t know what it is yet, I’m kind of figuring it out. I had the urge to just go make something spontaneously, like I used to do. I would have an idea and go make it, either a short film or an experimental idea."
No other details have been offered on the secret project; at this point, we don't even know if it's a short film or a feature. As to the other movie with Lowery that Affleck mentioned, that would be Old Man and the Gun, starring Robert Redford as a longtime bank robber, which looks to begin production by early next year.