i think i get this as well, i know i have a squeeky brake pedal but on occasions i get a squeek from the car when the brake is released (handbrake sometimes aswell), i'm think its the rear brake drums on mine as since i bought it they haven't been replaced and my fronts have had new discs and pads.
If its drums on the back you'll probably find theres no grease on the contact points for the brake shoes , take off wheels/hub/shoes etc and there are i think 2 or 3 for each shoe , lube up and all should be good , unless of course its disks in which case ignore all of the above