Hi
BIG HUGS
I am very sorry about your impending loss.
A distinctive 'death smell' has so far never been mentioned on here and I have certainly never noticed but then my sense of smell is pretty limited between a decade of chronic sinusitis and Long Covid. Like with so many of our own senses, there are super-noses like there are super-tasters. I can distinguish colour tones (and remember them correctly for years) long after most other people can only see a single colour. My mum is even stronger than me.
There is however a smell, a bit like nail varnish remover (ketosis) from the mouth that in guinea pigs indicates that the digestive system is breaking down. It is generally associated with advanced stages of pregnancy toxaemia (a metabolic disorder that can be easily treated in the early stages but that is fatal in the advanced ones - if you can smell the ketone build-up is usually too late).
The smell of ketosis has also been mentioned several times on here in connection with a heat stroke as one the of the potential symptoms of sufferers that haven't died immediately but are struggling with GI stasis and failure to get the digestive system going again and that may not recover.

It may be that this is what you can smell that in a dying piggy whose digestive system has stopped working and who is no longer able to eat?
However, my thoughts are with you and your old lady. I wish her a gentle journey to the Rainbow Bridge.