Two days later

"Hey, Doc, I just remembered something I was gonna tell you a few days ago. I think I know how Biff got into your lab to get the mind machine." said Marty.

"It must have been pretty clever because all my windows have motion security systems, cameras, and a key code on the back door. What do you think happened?" replied Doc.

"He got in the front door. The key was under the mat." said Marty handing Doc his key back.

It was the same key Marty had used to get into Doc's house the day everyone was under Biff's control. He must have put it back when he was done robbing Doc.

"I was smart enough to invent and build a time machine, but I left my front door key under the mat?" thought Doc to himself, smacking his hand to his forehead.

Marty smiled. Good ole Doc.