Squeak when break pedal is released

On my girlfriend's 2006 Pontiac Vibe she had been noticing a squeak when she releases the brake from a complete stop. I recently drove the car to listen to the issue. Here are the details:

It seems as though the squeak is coming from the rear (driver's side probably, but that could just be bias from sitting in the driver's seat).

The squeak occurs right before the break is completely released, i.E. If I am at a complete stop, the squeak does not happen when I begin to take my foot of the break pedal, only right when my foot is completely coming off the pedal.

It is a single squeak, i.E. The sound probably only last 1/10th of a second. The sound does not change in intensity if I change how hard or lightly I have the break depressed when I am at a complete stop.

The squeak happens when the car is in drive, reverse, and in park (including when the car is turned off). My girlfriend says the squeak occurs most of the time, but not always. I have not driven the car enough since it started happening to really determine how often it does squeak versus does not. However, on rides where it does squeak, it will squeak with each release of the break. For instance, the first time I drove the car after my girlfriend told me about the noise, when we got home, parked and turned off the car, I pumped the brake several times and each time I released the break the squeak happened.

Finally, this squeak does not seem in any way to affect the braking ability of the car (it is breaking fine) nor does it seem to impede the acceleration of the car as it is moving from the complete stop.

Any advice is much appreciated!