roo, in fog, there can be enough light diffused about that your car thinks it's daylight, especially if it is in the day! Not sure what Hoppy is talking about....
"I see plenty of people in fig who rely on their daytime running lights, seemingly unaware that this means they have no rear lights on. " - If even the normal sidelights are on the rear read lights will be on.
Most cars now have actual fog lights, designed to be seen, bright like brake lights but there should be only one at the rear.
Personally I never use "auto" on my lights.