2008 Ford Mondeo brake dust query

Hi, i've a two year old Mondeo 32000 km on the clock. I've recently had it serviced. My breaks have always been a bit 'squeky'. I noticed that when I was driving I was hearing a squeaking noise coming from what seemed like the left hand side of the car. The problem was intermittent but as the warranty is almost up I decided to get it checked out by ford. On collecting the car I was told that it was a build up of break dust that was causing the noise. Im just looking for a second opinion as I cant see how break dust could be affecting the car and creating the noise when im not actually breaking?
Any help would be great.
Thanks