Man you guys, I'm reading this thread with tears pouring down my cheeks.
My Jasper went from a fat cat of 14 pounds to a frail 7 pounds in a matter of weeks. He was throwing up what he tried to eat, and drank very little water. Frequently I gave the cats "wet food" (the canned stuff) as a night time treat. Every time a dish clanked, Jasper would be at the kitchen looking at me in anticipation for the wet food. Even in his last days, and though he wouldn't eat any, he would be in the kitchen waiting for it. Because he came running, the other two cats did too. I have a tuxedo cat named Bob who does this. It always makes me think of Jasper. It's like Bob wanted to continue the tradition. The day before Jasper died, Jasper was on the foot of my bed. Bob jumped on the bed and laid right next to him. This was very unusual for Bob. It's not that the cats didn't get along, but that Bob isn't the cat that goes to you or to the other cats, we go to him. It was then I knew that Jasper knew.
What we guessed happened was Jasper had eaten some sewing thread. It had gotten stuck in his GI tract and was rotting away the insides. If he had been strong enough, the vet could have done exploratory surgery to see where this string was. But by the time it was discovered that was the problem, it was too late. As to where he could have gotten the string, who knows? My only guess was from a closet or under a bed.
I've decided it's easiest when the cat runs away and never comes back. It's easier not knowing, because there is hope that they're still out there somewhere.