Silicone cures by elimination of acetic acid. Think some moisture is
necessary but if it got wet and acetic did not evaporate, this may be
source of curing problem. I had an old tube of their Silicone I and in
spite of careful sealing (Saran and aluminum foil) tip had hardened but
rest was still good. Part of problem is that sealant is in plastic
tubes and these tubes are not completely impermiable like glass or
metal.
Frank


RTV Silicone released a vinegar odor. My only problem with it was like
yours: when stored after it was opened, it would cure from the tip down.
I could salvage a lot by drilling out the cured caulk.

Silicone II smells like candy and releases ammonia and I think methanol.

Interesting. Must be different curing chemistry involving different
end groups. I cannot readily Google up but op's note would indicate
that if polymerization does not complete, you end up with viscous
silicone oil.