I can't see this video right now, but there are a few possibilities on what it could be:
1- When the film is developed, sometimes the reflexion of another film gets printed, I've seen this happening to photographs.
2- Sometimes a defective camera can reflect something that's not onscreen or leak light into the film (you can see this in "Godfather" and "Last Temptation of Christ", the later actually used the film problems as the ending of the film)
3- Some prop or crew on the background? (such an example of this could tbe the films "Wizard of Oz", and "Three men and a baby" which actually had a whole legend of the ghost of a boy who haunted the set, and it was only a cardboard prop seen later in the movie)