Sometimes when there is an impending renal failure, a cat will prefer to suffer crawling under a bush or even into a crawlspace to die when it senses that it is time. I lost an eighteen year old Tabby who died in my arms a couple of summers ago and that was the cause. One way or another it is hard to lose a friend.
I lost another Tabby in a place called Rattlesnake Canyon in Santa Barbara back in 1975. Her offspring were smart to stay closer to home and they outlived her by many years. One of those, Moki was hit by a car at nine but I knew what happened to him and that helped in the long term even if it was surely a bummer to find him. There was no mystery at least. Suzy his sister outlived him by four years, and she chose to disappear like your friend. That is a tough situation.
You have to figure we'll all get to the Elysian Fields someday and there must be a corner of them with lots of chamomile and catnip. We'll meet again with our purring furry friends in the great beyond.
Condolences
yogione