Back in July of 1986, Starlog Magazine printed an article by Bruce Gordon about "Back to the Future", in which Gordon ponders the idea of the alternate universe created in the Robert Zemeckis time-travel film. Gordon talks about the opening of the film, as we see Marty and Doc get ambushed by terrorists. Gordon speaks of a mysterious figure in the background of this opening scene, which takes place at Twin Pines Mall.
At the very instant that Doc tosses his pistol to the ground (and all eyes in the audience are following its path across the pavement), a silhouetted figure steps into that light. Less than a second later, the figure is gone, Doc has been shot and the chase is on.
I thought it would be fun to look at the artifact that Gordon supposes is "Marty II" in HD.
Clearly, it's "Marty II" back there. Or Bigfoot. Gordon wrote follow-up articles after "Back to the Future Part II" and "III" were released. They're available online via the Internet Archive.
“The Other Marty McFly” by Bruce Gordon, Starlog #108, July 1986
“The Return of the Other Marty McFly” by Bruce Gordon, Starlog #54, May 1990
“The Other Marty McFly Rides West” by Bruce Gordon, Starlog #170, Sept. 1991