[tcrlaf]

To me, this sounds like a gradual depressurization at altitude.
Did someone put the plane into a steep dive using the autopilot, realizing something was wrong, before passing out?
The no Mayday part is the most curious.

[tcrlaf]

Hypoxia at altitude is REALLY insidious, and hard to detect until it’s too late

This sounds like a steep, but CONTROLLED descent, that makes me think it was autopilot-initiated..