Help 2nd set of new brake pads squeaking

Z Bear

My wife drives our 2010 370z to and from work with easy highway driving and not much braking. At about 70k miles we developed a very loud squeal from rear brakes. Our mechanic seems to really know his trade and we trust his work and craft. He inspected the brakes and said they were getting close to needing replacement. I gave him the go ahead and asked that he use the Nissan stock pads. He turned the rotors too. We picked the car up and the very next morning when my wife backed out of the garage...the loud squeal was back. It happens every time the car sets for awhile and then is backed up.
I took the car back and our mechanic said he replaced the stock pads with Akebono's. We picked the car up yesterday and we are right back where we started ....still squealing when she is backed up. After eight years of braking, could the rotors metal be changed so that even with turning and new pads they squeal ? Please help ! My wife is embarrassed at work with squeaky brakes when she back out of her parking spot