I doubt the dog agreed to his own death
Odd that such an expensive house apparently didn't have CO detectors?
(Where I live, Netherlands, a smoke / fire detector is legally required on every floor of every house. Easiest for people to buy combined smoke and CO detectors)
I think this is because of the relative prevalence of unvented gas heaters in the Netherlands.
I live in an old house in Houston that has a gas jet in every room. (house was built in 1948)
I have one unvented heater for very cold days when my undersized central heat is overwhelmed...I keep a monoxide monitor in the same room as the unvented heater.
If it was monoxide for Gene Hackman, it was due to a faulty central heat or water Heater.
For the two dogs still alive, they may have been outside through a doggie door.
And if it was warm the faulty central heat would not run and the monoxide in the house would disperse