That was my first thought. A clogged flue for the fireplace or woodstove, or faulty heating duct. I also thought about food poisoning, but they would have to feed the dog the same food, which a lot of people do. The “No foul play” line is is premature from my perspective, if they don’t know the cause of death, how can they rule out foul play?
My gut says suicide. They had a recent picture of him and his wife. Yeh...I can see that as a possibility. No CO detector??