In the newer cars with IR lights in the rear view mirror assembly I think the cabin camera is used to measure attentiveness using face/eye tracking. It might also be the case for earlier cars, or indeed all 3/Y.I have been shown red steering 5 times in 3 days. Four times when I was wearing dark sun glasses.
If the car can’t see your eyes due to sunglasses it must think you’re not paying attention and then it no longer gives you the blue pulsing warnings about holding the wheel etc, it goes straight to the red warning and AP disablement.
I’ve read reports where people who have taped over the cabin camera for privacy reasons or whatever always just have red warnings.
That’s my working theory in this.