Nothing. I can find nothing wrong with my tyre or wheel. A complete mystery. I half suspected at 8.30 this would be the case and a of air would solve it. On the other hand I know someone who set off on a group ride middle of last December. On tubeless and got a flat, after other people had spent 30 minutes wrestling with the problem she confessed the valve was leaking when she set off. Another 30 minutes later a tube had been inserted and off the group went. 14 people had to hang about in December for an hour.
That's why I won't set out with a group if I have a potential problem. Very bad form.
So my Kinesis is clean, shiny, degreased, lubed, inflated and ready for the morning. The Cervelo was already shiny so I've prepared her for tomorrow as well.