My Girlfriend has a 2016 Toyota RAV4, 49,500 miles, she bought it brand new in 2016.

We noticed that her brakes make an awful grinding sound when she leaves it parked at my house overnight in below freezing weather. The sound eventually goes away after the brakes have warmed up. Now, we know this can be normal and the best thing is for her to park the car in a garage.   We've also made sure that her brakes and rotors are in good condition by asking two independent shops.

My question is:
Is the moisture that builds up overnight a threat to the rotors?  Physics kind of tells me that cast iron or steel beats ice any day.  I just wanted to confirm with the experts that the grinding noise is not doing any harm to her rotors.

Thanks!