The fog wasn’t there... and then it was.
It didn’t rise up, drift in, settle down, blow through. I thought I might have zoned out, but no. After I was done blinking in surprise, I turned to the person next to me, who wasn’t done blinking in surprise. There’s probably a scientific name for it, along the lines of ‘saturation point’ or some such. Because giving an alarming thing a boring name? Has a long history with making scientists sleep better at night.