That happened to me once last summer. I had a flat on a ride and replaced the tube with a brand new one. Filled it with CO2 and rode home. 2 days later I found the tire flat again the night before a ride. Took the wheel off, checked the tube and found nothing. It was then I remembered that CO2 will leak out of a tire quicker than air. Refilled from my floor pump and still riding the same tire and tube no problem.