If the wheels are old, get them to check on the inside of the wheel for mag "rust" (it's not red rust, it's white and looks a bit like limestone deposits). The "rust" can cause a poor seal between the wheel and the tyre making it seem like you have a slow puncture when there actually isn't a puncture in the tyre at all. If there is this "rust", they can grind most of it off to improve the seal. It's a cheap and quick fix. Takes 5 minutes or so and they did mine for free.