I take my car out once or twice a week for about 6 or so miles. Not only to sharpen up battery and push oil round moving parts, but also to grind rust off brake discs. This builds up very quickly, even in a garage. Leave it for too long and pads could be damaged and/or discs as well. If you have left it for a while, apply brakes gently to start with. Should be polished up OK when you can no longer hear them grinding.