I'll just add that I've observed this sensor working as advertised, although I was parked up with REST mode on.

Car in front started up and began to manoeuvre very slowly out of a space; lots of revs and visible exhaust. I heard the recirculation flap close and the pitch of the blower fan change. After the car moved away and the exhaust fumes had blown away, I heard it revert to outside air.

The recirculation lamp didn't light. It only indicates that the driver has manually selected the recirculation function.

It also says in the manual about this function only activating when there is no risk of windscreen fogging. Probably in winter there's little chance of it activating.