I was driving north or at least north-ish. When I looked in the rearview mirror, it seemed as though the car behind me had a light on inside: not the main light, but something near the dashboard. How odd!
I cast my eyes to the car behind him and noticed the same thing. Well, two cars wouldn't be driving with interior lights on, so there must be an alternative explanation.
Once we had completed our errand we turned back the way we came, and we saw that many approaching cars were exhibiting a similar effect.