2008 Kia Sedona OCCASIONAL NOISY BRAKES

When my wife goes to work, the van is setting for about 4-5 hours before she goes to lunch, when she leaves to go to lunch the first 3 stops in the parking lot is when the brakes make a grinding sound and never returns until the next day during the same time. I just rotated the tires and saw nothing wrong. The other day it was raining and only then during her lunch it did NOT grind? What the heck could it be? Very strange.