Sometimes, I have been round the inside of a tyre with my fingertips, square millimetre at a time, and never felt or seen the offending bit. Sometimes they just withdraw back into the rubber when the tyre pressure is off. When that happens you just have to go over the outside of the tread and investigate every single tiny nick in the rubber. A small safety pin is ideal for digging the offender out. Sometimes, I have never found the sharp thingy and, through boredom and frustration, just risked stuffing on a spare tube anyway. Sometimes it does work.