I prefer to change my flats slowly and systematically . Seeking out where was the puncture and why. I prefer some preventative maintenance to avoid flats on group rides....
. Sometimes small , embedded fragments of glass or whatever are hidden within the tread. Unless you take a close look you might miss it..
After 3 slow partial leaks I took apart the tire to find out why.. All looked well. Nothing embedded in the tire . I put the tube under water. No leaks are to be found anywhere. So, I used it again.. Next day a flat. Afterwards, I again submerged the tire in water. No apparent air bubbles after a prolonged time under water.
I changed it anyway.
I'd call that a mystery flat.