So you took a puppy that has a collar and tags, and drive it 50 miles away and trying to find a new owner. I would have taken it to the vet to see if it was chipped
I was thinking, if that is older it is probably triple wall pipe. Which is air insulated. Cold air travels down the pipe and gets trapped in the chase under the firebreak. Has no where to go but through the cracks into the house. We used to put cold air kits on fireplaces that did this...