squeaky brakes after rain or carwash

2013 Prius V, 160K. We recently replaced the fluids, brakes, and head gasket. Head gasket was a biggy.

Anyway, we noticed that the brakes squeak lightly for several miles after some rain even while brake pedal is not depressed even if the car was parked inside during the rain.

We have heard of several sources of this noise, including light rust on the rotors, which doesnt make sense: the rotors on my pickup do not squeak under similar conditions

Is there a definitive causes of this annoyance?