In every story that has ever been written, or ever will be written, there is a six-year-old girl with the main characters. She is small and quiet. She never says anything and nobody talks to her. Although she does follow people everywhere, nobody pays her much attention.
In all of our stories, we never mention her either. We never describe what she looks like, what she's doing, what she sees or looks at. In fact, there is no mention of her in any story that has ever been written (nor will there be mention of her in any stories that will be written).
Still, that girl is there. How can you prove that she isn't? Just because an author doesn't mention a person in their stories doesn't mean those people don't exist. If a bed is not explicitly mentioned, we still know that it is present in a bedroom. We assume all homes and offices are furnished and well-stocked on common items. We assume that every business has workers maintaining it. We accept that there are a great many things that exist actively in stories we read, despite them not being mentioned. Why not a six-year-old girl?
Imagine if this was true. For one thing, it would spooky as hell. For another thing, it means every film adaptation of a book is wrong (unless we assume that girl is just out of shot for every single scene). But, imagine if a film adaptation of a story did have a six-year-old girl in it who was never mentioned or acknowledged and has no lines, but is in every scene. What a great way to screw with people's minds.
Writing can be tricky like that. There is a lot going on that exists only in our minds. When you start toying with that, you expand the realms of possibilities in your art.