Hmmm. In areas out West I used to get a lot of flats from goat head thorns. Usually I'd find a tiny piece of the thorn embedded in the tire. It was often tough to find but by matching up the tiny hole in the tube with the area on the tire under a bright light I could find it and dig it out with a sharp tool so it didn't cause another flat. Out East, I regularly get flats from tiny pieces of wire or very small slivers of glass that get embedded in the tire and work their way through the tube after many rotations. These can be tough to find, but again, by matching the hole in the tube with the area on the tire, using a bright light and a magnifier, I can find and remove them.