I examined the tire closely -- nothing. It felt smooth inside and out. Then I pulled out my trust awl and probed the hole. As the astute reader has guessed long ago, out popped a piece of glass. The glass was small enough not to protrude from either side under examination conditions, hiding completely in the thickness of the tread, but it was large enough, apparently, to poke out and cut the tube under riding conditions.

Lesson learned.

Now when are my Marathon Supremes going to ship?