However, it is likely that a 2-pack airplane is allowed to dispatch with 1 operable pack. In that case, a second failure is not unforeseeable...
Normal leakage will allow slow depressurization even if the outflow valves are closed.
The ram air valve installed in some (e.g., 747 Classic) airplanes may give a few tenths of a psi of differential pressure, but that's all.