Perhaps there is a blockage, due to hairball?
One of my cats, about 15 years ago, stop eating, had very high fever, not moving and just sit there. I cannot remember all the details, but nothing show up on x-ray or ultra sounds. Eventually he was operated and the vet found and removed a huge hair ball just near the esophagus. Nothing else. And he recovered (unfortunately in 2003 he die due to the same problem. This time due to negligence of another vet even after I told her that could be the possibilities. He die a very healthy cat and young).
PLEASE do ask the vet. I cannot remember if Whisky salivating. But he had diarrhea and vomiting, high fever, listless, and eventually just sit and not moving.
I just checked with my Subang vet, he too said he had done a post-mortem on a vet's cat and it turned out to be hairball. The vet had not considered this possibility as well.
Gosh, could we have overlooked this in Indy?
My vet said the Barium Meal (Barri Meal) would help in this case. That would be less risky than opening Indy up to check if it's a hairball problem.
I'm going to suggest this to the attending vets and see what they say.