Dodge - Caravan :: 2012 - Left Rear Brake Began Making Grinding Noise Only When Applied

I have a 2012 dodge caravan bought new. After about 6000 miles the left rear brake began making a grinding noise, only when applied, every time the car sat more than a few hours, that goes away after about 2-3 miles. It began in winter and continues into the warm and dry days of spring now with 9000 miles total. Had it to dealer twice and inspection found no malfunction so they say its from rust build up because it goes away all the time and is nothing to worry about. It does not occur with the other wheels. My niece has the same van, in same geographic area and does not have this problem. Does this selective rust build up explanation make sense? Previously burned with bad brakes on a caravan, so am skeptical.