Moisture/misting up inside the reverse lights this morning!

[scoss]

Spotted this when I was scrapping the car today. On both lights in identical places in the corners was some misting up. I'm talking about half an inch strip. All other parts of the lights were fine. I know it was -2 so I'm keeping this in mind but should i expect to see slight moisture in this weather?
I know the rear lights aren't as incased as the fronts. Appears to be a simple plastic cover to access them.

Thanks