Something caused the pinhole. Maybe it was a defect in the tube that finally opened up a pinhole, seems unlikely. Maybe there was a tiny piece of somenthing you didn't
find that caused the pinhole. Maybe one of the spokes is just barely pushing through velox rim tape, seems unlikely too. I have had flats where I never found the cause also. I go to the trouble of wiping and vacuuming out the rim when changing tubes.