I have a slow leak in in one of my tires/wheels. Took it to the tire store.... says no leak. So why does it loose a lb of pressure every week?
I've found that you have to get them to check it the second time. The second time around they always seem to find the leak. I had this happen once before. Took it in the second time and they discovered it was corrosion on the rim. Depending where the tire/wheel's position (when parked), it would sometimes leak air, other times not. At least that's what I was told. They removed the tire and hand-sanded the lip of the rim smoothing it. It solved the problem. This particularity rim was on a 10 year old car.
If all tires are responding alike then it is typically an air temperature condition (1 psi per ten degrees F* change; up/down). If one wheel is losing pressure then it can be a variety of things, i.e., puncture, air valve, porous wheel, poor mounting, etc.