CO poisoning comes essentially without symptoms. If a furnace with a leaking heat exchanger is running, or a flue vent in an attic is leaking in a place where it can be drawn into the return side of the furnace, you are going to spread CO pretty uniformly throughout the house. That is why even just a single working CO sensor in a house will provide a good level of coverage...especially if it is near the main return grille to the furnace...unlike smoke detectors, which tend to be needed in more than one location to get good coverage. Of course, if you have several smoke detectors that also detect CO, you've got it covered.
The death of the dog makes it pretty clear that it was not drugs/alcohol/etc. I'd give good odds on CO.
CO can also cause unconsciousness/death
very quickly - in just a few breaths, if the concentration is high enough. It's plausible that it could cause victims to suddenly fall down. Effects would presumably be worse on the elderly.
Gas company has already said they haven't seen indications of a leak, but if it was a faulty furnace, the gas company might not find it. They're only going to look up to the meter, or maybe to the furnace intake. If the furnace itself is malfunctioning, the issue will only appear when it's operating (obviously) and will be worse when it's under load. Depending on the level of malfunction...that could easily be the culprit.
Not sure if the dog is really an indicator, apparently there were 2 other dogs that were alive. I'm wondering if maybe the dead one ate some of the medication that the wife dropped?
The open door is still a puzzle too.
Edit: Just read a new article that said the dead dog was in a crate. Seems like that makes the CO theory a little more plausible. The other two dogs had access to a doggy door. And, this article said that while the initial reports said no foul play was suspected, authorities have walked that back a bit. They're now saying there are no obvious signs, but they can't rule anything out.
Incidentally, regarding CO detectors - a lot of people use them wrong, with encouragement from the industry. There are lots of plug-in detectors that you can mount in a standard outlet. Problem with those is that the typical outlet is about 18" off the ground. CO is lighter than air, so will tend to collect near the ceiling. Pretty much everyone breathes more than 18" above the floor, including when they're sleeping. So, you're going to be breathing the cloud before it touches the plug-in detectors.
