Had a similar situation once. Turned out to be a sharp rock that was only exposed when the tire was inflated, likely picked up on the Towpath. When inspecting the tire with my finger and cotton ball, there was nothing. I saw a tiny little disturbed area on the inside, and pushed a nail through there. Lo and behold, there was a good size rock embedded in the casing. As we both ride in the same area, this could your issue, as well.