All planes have an altimeter that shows the planes altitude. Pilots are trained to know the psychological limits associated with flight. In this case the pilot and passenger would have been using supplemental oxygen at the higher altitude. That onboard ox supply can and does fail occasionally. Pilots are supposed to be trained to recognise the onset of hypoxia. But it can be insidious. Another possible scenario involves an exhaust leak getting into the cabin. But that is not likely in this case.
Thank you for answering my question! I appreciate it!
A follow up: What is the advantage for that sort of craft to fly at the higher altitude, knowing there’s a risk of succumbing to oxygen derivation?
The temperature has been frigid (32F+/- by day and 22F+/- by night) for the past few days, but today was the first day of clear weather for about three days. It had been rainy with the temps near freezing, so I suppose if the craft were lost on Monday, it might have been trying to stay above the freezing rain to avoid icing on the wings?