It was semi foggy this morning on my way to a client site. Some drivers had no lights on at all. I had mine on (front and rear) but no fog lights. It got a little foggier and the van in front put his lights on, then his rear fog light, but turned it off again straight away. Good chap, I thought. You clearly looked in your mirror, saw me and thought "yes, I can see the car behind, I don't need to fry his retinas with my fog lights, and potentially cause him to run into the back of me because he can't make out my brake lights due to the glare". Five minutes later they went on and stayed on. Even when the fog cleared they were still on. Had I been a Rozzer, I'd have given him a tug.