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Cats don't really go off to 'die'. If they don't feel well, they don't want to associate with anyone, so they will go somewhere that they will be left alone. The problem is, that you can't treat them if you don't know where they are and if they are ill enough, they will pass away. Expand your search to look in any secluded areas around the house and get her into the vet when you find her. I'll say some extra prayers for you that she'll turn up soon!