I [censored] up

[n.s.]

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I'd say there's a very good possibility that he crawled under there to die before you closed it and that you didn't do anything at all.

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Hate to just QFT something, but this is very likely the case. Don't beat yourself up over it, especially considering the age of your cat and the effort you put into finding him. If he wanted to be found he would have made himself heard, or, in healthy catlike behavior, gone sprinting out of the door the moment you opened it. The mere fact that he stayed probably meant it was just his time.

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I agree. If he wasn't making all kinds of noise when you were poking around in the crawl space, it was probably already too late to help him (and it may have simply been his time, at his age).