Lost Painting in Stuart Little
This is less of a hidden message than an (unintentionally) hidden object. In the 1999 film, Stuart Little, the main characters are an affluent Manhattan family who happens to own a lot of nice paintings. In 2009, an art historian named Gergely Barki was watching the film with his daughter when he noticed a painting by Róbert Berény in the background. That particular painting had been missing since 1928 and was worth around a quarter of a million dollars. Barki eventually got in touch with an assistant set designer for the movie, who had bought the piece at an antique store in Pasadena for just $500. And the painting was returned.