I wonder if the Helios crew thought that the audible horn was a false 'take-off config' warning?
From my memory of the report, yes, they did, and this was indicated by their communications with their company's engineering staff.
I remember the Australian flight safety journal, quite a few years ago, having a special issue devoted to hypoxia incidents. It contained story after story of incidents of non-pressurisation that were detected by the crew, just in the nick of time.
Young readers of that edition of the journal could have taken away the impression that, one way or another, you would detect when hypoxia was setting in, and then take action to save the day.
To me, a glaring omission from that edition was some sort of great big emphasis that luck played a part in the detection of the hypoxia, and that luck cannot be relied on!
I think they should have featured a number of cases where luck didn't present itself, with the incident then going on to become fatal for all concerned.
When discussing pressurisation with young pilots I always endeavour to describe the risk as something that can quietly sneak up on you from behind. Let it get close enough and it will snatch your life. You won't know a thing.