I have a similar experience, but it happens when I look at people in a car next to me in traffic, or waiting in line somewhere, and I see a tired face, or a stressed-out face, just a plain-ol-face, and think, "I don't know you. I don't know a blessed thing about you. But who knows? You just MIGHT be the type that piles into a car once a year to sing your heart out with like-minded weirdo-folkie types. Who the hell knows?!! My God, you might do it every Tuesday night!"
And then I have to smile at them. And they smile back. And then, I think, maybe I DO know you ; )