Originally Posted by Kyronea
That scene always was a little weak. Marty should have walked completely out of sight of the DeLorean. (Which technically should also have been parked further away from Old Marty's house. Old Marty would have recognized it for what it was and it was dangerous of Doc to leave the DeLorean nearby.
Then again...it was before Doc learned all the lessons he learned about time travel safety...)
EDIT: Oh my goodness...I actually spelled scene as seen...I deserve to be whipped for that...
It woudn't have been a big deal if old marty did see it. From his perspective, he'd remember the trip to 1955 (there's a fair amount of evidene in 2015 that these events did happen) and know what happened. He wouldnt remember his trip to 2015 as this is happening now. The danger is him seeing his younger self.