This supposedly occurred Monday night after he endorsed Hillary at the convention, so there would seem to be little reason for it. If it happened, I can’t believe that Sanders or his aides would cover it up. It’s something Drudge would jump at if it could be verified.
On the other hand, Sanders said he was going back to being an independent when he returns to Congress and this may have prompted that decision.
On my computer..if I look close enough...I see that same spot on the Upper Right photo.