You are describing a binding caliper. You have an xi, so you could have an unrelated problem to the brakes that is really about the front drive train. An inspection of the front end of the car should uncover the problem.
I'm not in complete agreement with Zell, I think that if you replace one, you should replace the other.
Having said that, cleaning the pins that the calipers ride on, and cleaning the holes that the pins go into will go a long way to correcting a sticky caliper most of the time. If the problem is the brake caliper or the front drive train, it will require taking the tire off to see. Since the car went to the mechanic, there's not very much you can do with it now.