Some wheels, my Volvo 740's aluminum wheels for example, can leak around the bead - the place where the tire meets the wheel. It takes 2 or 3 weeks to lose 15 lbs. I take it to the tire store I buy all my tires from and they fix it for free. They put a soapy water solution around the bead with the wheel laying flat. A small bubble appears that takes several minutes to form! They take the tire off, buff the oxidation, dirt, grime, etc. off the rim at the bead and remount the tire. That usually fixes it. This has happened 6 or 7 times in the 15 years I have had the car.
The owner told me that it is fairly common, they see this several times a week. This place is big volume and probably sells 100 - 150 tires a day. He specifically said that the WORST of all is the new Cadillac chrome plated wheels. He said that about 50% of them come back with bead leaks.