Dropped off a passenger in misty conditions. When I restarted Bong, Turn off rear fog light. Big symbol on dash. Didn't want to turn it off, but off and on and warning went. Later on the same warning. I ignored it and went away after a few seconds.Guess it's to prevent usage in clear conditions. 

But first time for me in 4 years of ownership . Simply clever?

Possibly. Few people realise that it used to be a moving traffic offence to have a fog light on if the visibility wasn’t below 100 yards. At least I think it was 100. That applied to the front as well as rear.... it might still be, but I’m sure traffic cops have better things to do. How the car detects the visibility I’m not sure! 🧐 must be super clever....

Out of interest then are cornering fog lights legal? I've nearly lost count of the number of curbs I've run into when turning because I didn't see them - Let me think - Ah yes, None, but somehow the manufactures convince us we need a new super feature to differentiate the new model from the old... 

To be fair the fog lights are awkward to use, I'd far prefer a separate switch to put them on and off. It would be pretty easy to leave them on by mistake but can't say that I've ever seen that particular warning. Although the Front Assist warning regularly goes off when it picks up someone on the other side of the road on a country road that I drive to and from work by! It was pretty bad on my first Octavia mk 3 as well.