Thank you all very much. It has been a really, really hard 24 hours for me. I thought that I killed Dante. Hell, I still might have. But everyone I've spoken to today, including animal rehabbers and vets, told me that there is basically no possible way he was healthy and trapped for any significant amount of time and I don't hear him. All the vents were open in the crawl space, and I live in a quiet space, and went outside many, many times calling for him. Everyone has told me that he had to be sick, and went off to find a quiet place. They told me he probably pulled down the insulation not because he was trying to get out, but because he was trying to make a nest.
I am still broken up. I wish that I had just gone under the house that day I noticed the crawl space door was open. Even if he was sick, he might still have been alive. I might have gotten him to the vet in time to save him. Now I'll never know.
But I don't believe there was any way that he was trapped and healthy and I didn't hear him. He had to have been sick, sick enough to find a quiet dark place to pass away.
Dante:
Goodbye old friend. I miss you terribly. I'm sorry I didn't find you in time, and that you had to go all alone in such a cold, dark place. I'm so lucky I had you for 13 great years.