I was talking to a friend today who said that she noticed one of her tires was losing air regularly. Assuming it had a leak from a hole she couldn't find, she filled it with air and made it to the nearest tire center. She bought a new tire and had it put on, only to have the service guy tell her that nothing was wrong with the old tire. She realized later that it was that the rim of the wheel was rusting and letting air out at the edges of the tire. Wow! Might be something to check if your tires are losing air.