[cornflakes]

Here's another possibility: they may have or you may have wiped the inside part of the windshield with some cleaning agent and though it looks clear when dry.....in the winter the invisible film will turn into frost.

I had wiped the interior of my windshield once and never noticed anything until a few weeks later on a cold day, there was frost on the inside of windshield and i realized it was the cleaning agent (the residue) that had turned to frost.