well i have had hexi in the garage for years and it does go off, if you like, but thats not stored air tight, the only real answer is if your worried dont do it, the gel how ever should be fine as long as its sealed, just give it a good squish in the packet before use to ensure its mixed well. you might find the tabs have sweated because of where they are stored, keep them away from any heat such as radiator etc etc, try to keep them around 15-18 c and they should be fine though...
let us know how you get on though,as the sweating is fuel being lost, as an alternative you could make some waxed cotton balls, they will last for ever, almost, and wont go off as such and are waterproof untill pulled/fluffed up to use...
hope you get on ok..
chris.