One point is the plastic all over the dash, and doors has a rather dull finish. If yours had a shiny finish, the dealer or some person definitely added a treatment to the surfaces. Plus the comment the car out in the sun makes it worse:
A silicone type product is pretty hard to fully remove. So I would say wash all the plastic down with a dish washing soap solution again, at least a few more times.
One thing about the stupid dash top plastic is the surface is not sealed. So any chemical put on it will be sucked into the surface. Washing it off will take some effort.
On my 2012 I used a Meguiars product as the dash top was flat (zero gloss) and felt odd. The liquid clearly soaked into the surface!
On my 2014 it was less so.
Anyway, if the problem car came with a shiny plastic dash top, that is NOT from the factory.
Maybe some idiot spilled a product on it during build.. And they decided best thing to just rub it all over until it looked ok?
Anyway, wash wash wash... Particularly the dash top and upper door plastic.
I would use a washcloth, a cotton one for hands.. Not a sponge... And wash it every week for a month.. Then if it still stinks? SOL. [mecry]
If it was an small mammal like a mouse or squirrel, the stink would be like severely rotted vegetables in an explosive stench when the gut explodes from rotting.. That would last a few days, as the animal dries out, the stink goes mostly away... So if it remains the same for weeks.. it is NOT a small dead animal. (sorry for the gross description, but hey, dead is dead.) A big animal takes lot longer.. but an animal the size of a dog or cat you will find it.