Reminds me of the time when I lived in Santa Barbara. I had parked my 1957 Chevy on a bluff near the ocean while I was in a building. When I returned to the car in the evening and started it the windshield turned opaque and was fogged almost instantly.

I defrosted it and it cleared and the next day I found that there was a pinhole leak in the heater core. Fortunately it was easy to remove the core (unlike so many modern cars) and the radiator shop fixed it for little money.

The effect is not something new and exists whenever you are using a heating system that has water surging through the heater core.

 

But these days early warning more often comes with a distinctive odor of engine coolant/Anti-freeze.