At exactly 1:02:00 in the movie, which is during the part where Marty and Doc are about to go back to 1955 after having discovered the details from 1985 Biff, if you watch the window behind Marty, there's is a mysterious figure hovering outside!! It looks like a camera, but it's hard to tell since it's dark outside, and it can't be Marty's reflection since it's a square shape.