Are you sure somebody is not causing the flat as a windup?

Also as mechanicalron mentioned, tiny wires can do it. I had one that was around 2mm and only found it after it flatted me 3 times. I had to use my tiny swiss army knife tweezers to get it out. I missed it time after time, it was in one of the knobby bits of the tire on my MTB. It only pushed through when pressure was put on it from the outside.