Is it the plastic decaying? I forget the product of this process (Butyric Acid?) but it really stinks.
BT
Hmm.. The plastic tub is old and its been stored in humid places, are plastic eating bacteria at work? Or even fungi?
I do know that plastic does decay, indeed, certain plastics (and also latex paints) are often eaten by aspergillus fumigatus (a filamentous fungi that grows also on wood, and the world's 'most successful fungal pathogen') producing abundant respirable toxic alkaloids when those fungi sporulate- These spores contain huge amounts of a dangerous chemical which has many unpleasant health effects.)
I am pretty sure that GABA-like chemicals which are likely related to butyrate which you mention, also may have functions in the body.
Many of the chemical products of putrefaction produced by bacteria have functions and an ancient history of serving as messengers in living things. For example, sewer gas, H2S, is emitted from the Earth during volcanism and is abundant around the volcanic vents deep in the ocean where many unique forms of life still live, its theorized that many kinds of life evolved around these vents and fumaroles, etc, long before our type of life did did here on Earth. H2S seems to be one of the threads that ties it all together, and still seems to have some interesting functions in living things, in regulating metabolic processes.
Now H2s is a subject of intense study (don't try this at home!) because its been discovered that it can be used to put lab animals into a state closely resembling hibernation for long periods, during which they require very little oxygen and many destructive processes that cause injuries in people's bodies are also prevented - The hope is that we will be able to put people into suspended animation pending their access to medical care.
Certain species of animals hibernate and the possibilities of putting people into hibernation hold out the promise of saving a great many lives in areas without access to medical treatment, allowing people to be transported to places where they could be treated, without dying on the way. (Also, it would give families more time to line up financing).
This would be a very exciting new development indeed, as hibernation could be used on long space flights to conserve resources during trips spanning very long distances.
Its also another example of where Stanley Kubrick may have gotten another future technology almost exactly right in his classic sci-fi film, "2001 A Space Odyssey"