Mine does it too. 37K miles. I thought it probably needed a brake cleaning service, just haven't booked a service center appt yet.
Just go out and do a couple hard stops.
Probably just surface deposits on the rotors.
Mine has been doing it while only reversing. Before the rain started when it got coldeR
Mine started doing this exactly under the same condition, colder weather, only while reversing.
After reading comments about deposits from very little use of pads, I think it's because I use the pads heavily for autocross, but the autocross season ended in October and now I pretty much just use regen except while parking the car.