As others have mentioned, tubes are porous and let air gradually diffuse through. However the matter is how quickly. More narrow tires will lose air a tad faster than wider - issue of tube surface to volume ratio. In my experience 2 weeks is often enough to make it necessary to replenish air in a healthy tire to keep it pumped up. However, if the tires went flat over such time I would look for some problem. If a tire went flat over few months, something could be wrong, but you would be unlikely to identify the leak releasing so slowly.