How many movies can you name
with this theme, more or less describable as "Are you sure the person you say is missing isn't a mere figment of your imagination? No one else can remember seeing them?"
Off the top of my head, I can think of three: Dangerous Crossing with Jeanne Crain, Bunny Lake is Missing, Carol Lynley and Flight Plan, Jodie Foster. I know I've seen it in other movies, a few TV movies for sure.
Funny how in spite of re-using a formulaic plot element, these all turn out to be pretty good movies.