If you pumped up the original (leaking) tube without reinstalling it first and saw that it apparently lost no air over a day or two, either the valve is releasing air slowly or (much more likely) the tube has a very small puncture resulting in a slow leak. Check the pressure with a gauge, and you'll find that there's not enough pressure to register. If you were to reinstall that tube and pump it to full pressure, it would lose pressure at exactly the same rate as before.