After I changed tires to conti 4 seasons in my road bike, the front wheel has been leaking air very slowly. I've changed tubes some 3 times already, and it keeps happening (after a couple of weeks). The tube will keep some pressure for about 3 to 4 hours before it's noticeable. Submerging the tubes in water doesn't help to find leaks, and the back tire is fine. I checked several times and found no sharp objects inside the tire.
The only thing I did find a bit weird was that the rim tape was not exactly on the center on one point, and would not stay in the center easily (it kept climbing the inner wheel).