Sorry, Sue, maybe we're not done with this topic yet. A bit technical here...

I'm not a chemist so perhaps someone with a chemistry background out there can give me a thumbs up or down on this idea, but I thought that a slow reaction of HCl with Na+ ions gradually degrades the disinfecting power of bleach by reducing the concentration of hypochlorous acid (the source of the HCl), irrespective of whether any chlorine gas is allowed to escape from the bleach container. In this way, even an unopened, sealed bottle of bleach can gradually degrade over time. Comments?

By the way, here's a webpage from Cornell University with a response from Clorox about stability. It doesn't say whether we're talking about opened or unopened, but it does state that the concentration will diminish.