Had an odd problem the other day, could here my back offside brake squeaking all the time. Had a look at it through the alloy and the disc looked a bit grooved. Whipped off the wheels and found that the the outer brake pad was worn down to the squeal pads but the inner still had plenty of life on it.
Google informed me that they had this problem in America, apparently rust meant the outer brake pad was constantly dragging on the disc. As the car was out of warranty, mazda weren't prepared to fix it. I did get a new set of pads and clips from them which were modified to stop the problem occurring again. Had to fit new discs as well but hopefully this won't happen again!