How does one die from carbon monoxide without knowing? Unconscious?

You Fall Asleep. Carbon monoxide binds to hemoglobin in red blood cells much stronger (with higher affinity) than oxygen does. Once it gets into your system, it starves your body of oxygen, and you fall asleep. If you're already sleeping, then you slip into an even deeper sleep. Eventually, all your hemoglobin is tied-up with cm, and there's no room for oxygen to enter your bloodstream. This is why you suffocate.