Disney and Pixar are known for sneaky details thrown into their movies.
Sometimes they are known as easter eggs, an intentional joke, hidden message or image, or secret feature hidden within the movie. Sometimes they are just clever details in the movies that you wouldn't necessarily notice at first but would make you smile if you did.
They run the whole gamut; some are appropriate and some are a bit on the adult humor side of things. There are tons of, shall I say, conspiracy theories about old school Disney moves like The Little Mermaid and details that illustrators drew in.
Either way, these kinds of things are what makes these movies so fun to watch, as you can spot something new each time.