Have you tried turning the tire inside out? You could have a tiny piece of wire from a shredded car/truck tire that you can’t feel with your finger.

Several years ago while on tour I woke up to a flat tire. The leak was so small I had to run the tube through a convenient puddle to find the puncture. Once I did I matched it up with the tire but still couldn’t feel anything until I turned the tire inside out. That’s how I found the piece of wire. It was maybe 1/8 of an inch long.