[He's been so preoccupied that he didn't feel their presence. He touches his forehead, and sure enough...]

How on earth did that get there?

[Once he's sure Marty's okay, he moves to stand in front of the crystal, the nearest reflective surface. Sure enough, what's embedded there is a bright crystal.]

I can assure you it's not a fashion trend from any future I've seen. I certainly didn't have it a minute ago. Did anything unusual happen to you?

[His voice is pitching up in that way it sometimes does when he's starting to worry. He turns back around---well, Marty doesn't look any different.]