Gryder mentions the Payne Stewart plane was reported to have frosted windows - covered here:
https://en.wikipedia.org/wiki/1999_S...t_interception
On the other hand, the windows on the Helios flight were all clear enough to see the cockpit with slumped pilot and then the entry by the flight attendant on oxygen bottle and the dangling oxygen masks in the passenger area.
Both aircraft had previously reported for problems with pressurization.
I'm not sure the passengers in the instance of a medical problem would fail to notice the additional time, nearly an hour, in the air no matter how distracted. All falling asleep is possible, but it's not that long a flight.
The current report is that ATC failed to establish contact about 15 minutes after takeoff, about the time for hypoxia to set in but, sure, stroke or heart attack could happen any time.