Just had a flat and having problems finding the thing that pierced the tube in the tire. Normally I just run my fingers around in it, until I found it, but this time everything looks O.K.
My experience is though, if you do not find the object, another flat will happen soon ...
I know the trick with searching for the puncture on the tire in the same space where the hole is on the tube. I even flipped the tire so that the pattern is inside, but nothing to be found.
It's a mountain bike tire, and the hole in the tube is very tiny.