CO poisoning is a real threat to most people, even when camping outdoors. Indoors, always have detectors, replace every decade, be aware of early symptoms such as headache, narrowing of vision, disorientation. If CO poisoning is even suspected, get to fresh air quickly, then re-evaluate. It slowly robs you of ability to think and act.
But alarms don't always help even if you have them. CO detector sensitivity degrades over time, and people neglect to replace them. Some people have poor hearing and/or sleep too deeply to hear alarms. CO poisoning is insidious, reducing both physical and cognitive abilities to take action. Dog w lower body weight wd have been overcome first. Wife may have died last, ignoring her own symptoms to attend to dog and husband.