We had a skiff of snow overnight. As I was looking at my Kona out in the driveway, I noticed that the 12V charging light in the grille was on. Two small rectangles of snow on the windshield in front of the rear-view mirror began to thaw as I watched. The photo shows that a third was also generating heat. The outside temperature is -4C. I don't know what this means but I'd guess some of the sensors behind the rear-view mirror were active.
I've seen the same behavior with that windshield sensor on Teslas. I'd assumed that was an attribute of Sentry Mode.
That's the sensor cluster for the Forward Collision-avoidance and Land Keep Assist features. It seems to have dedicated defrost heaters for obvious reasons, as well as a mind of its own.
It's a camera that does a lot of processing for the lane following etc. It may have some active defrost heating as well, but I suspect most of the heat is just what it's using to do the computation.
It may also be that there was a reflection of light from the camera lens back to the glass surface which caused some melting.
