I have a tub of star san in the garage that has been made up around 2 months, I'm sure I've sanitized a plastic mesh bag that had previously soaked in PBW and rinsed. Or does it just start to smell astringent or something after a while? Could it have picked something up from the plastic tub or any of the SS kegs its sat in? The pH is still low so I'm going to keep using it for now.