I have the same squeak, I think from the right side. Braking (normal or handbrake) does not seem to affect it, and it's only noticeable at low speeds, but annoying indeed. It was faint at first (I've had the car for less than 2 weeks), but some days ago it got noticeably louder.
I called the dealer, and they said the most common cause of squealing is some debris lodged in the brakes, typically between the rear discs and the disc shields. They suggested taking the wheel off and waving a small screwdriver or such around in the gap, but I've yet to try it...might as well have them to take a look instead.