At the beginning of the movie, Mike has another dream, except this time it's about something different. Everything's dark, and he can't see, and he hears musical boxes, and a voice telling him to come find them.
The next morning, Abby and Mike go visit Vanessa in the hospital and she's awake. She goes to her father's house now that he's gone and they look through his stuff together since she too had weird dreams in her comatose state. They find a box of photos and look through them. To their shock, it's all photos related to Fazbear's, pictures of the animatronics, hand-written letters to William by an unknown writer and even a picture from the opening of Fredbear's, which none of them knew about. However, when looking through these earlier ones - some ripped apart or even burnt, making certain faces unrecognisable - Mike starts to hear that same voice again.
"Come find me, Mike."
Not sure how the rest would go though.