I'm a bit surprised by the number of experienced and highly-respected organisers on here who are critical of riders who fail to notify if they DNF. Most of my audaxing was done in the good old days before riders carried mobile phones, and in fact even more recently when acting as a finish controller we have never published an 'event phone number' and don't expect to hear from DNFs. Its good when we do of course, but we know we can't rely on it. And you'd perhaps be amazed how many organisers or finish controllers don't really know how many riders thay have 'out there', so on that basis we have to stay to the end but (in our case) not a minute longer.
I'm also surprised that many organisers don't seem to use modern comms to track the fastest riders - on a long event they could delay the opening by hours if they know what sort of schedule their fastest riders are on. More useful than knowing about the slowest, I would say.
1. I have a duty of care to ensure all riders are okay, within reason — if you abandon without telling me then you're on your own, but otherwise it's part of the role;
I would say the same (my bold). Sometimes what we do when we close up if we think riders are still 'out there' is to drive back along the last 5 miles of the route, before heading home. Just to see if there's anything to be done.