Carbon Monoxide is slightly less dense than air, so it does not particularly “rise or fall” in air, but diffuses within air. If some time has passed, it is not unusual to find dead animals or people, but with just trace amounts of CO remaining in the air.
Poisonings are so common in Utah, that it has been nicknamed “Mormon suicide”. Often, a car is caught in a snowstorm and its occupants light charcoal briquettes to keep warm. They are found a few days later, dead of CO, with extinguished briquettes in their burner, and their bodies frozen. Only trace amounts of the gas remain in their car, though.
People in snow country, do not fire briquettes in their cars. Cite just one instance, of such an event.
The problem is that CO poisoning is something that can be seen physically on a body, blue lips,petechiae in the eyes..since they already ruled out CO my guess there bodies had no signs.
I've lived in Utah most of my life and have never heard the term "Mormon suicide" or that poisonings are especially common here. I guess I must live a pretty sheltered life.