One time, I had a slow leak that I could never figure out. Sometimes it would hold pressure for a week, other times lose air overnight. I did the water test (no bubbles) and wiped the inside of the tire with a tissue to see if anything grabbed it and torn the tissue, again nothing. The rim tape was also good.

In the end, I just bought a new tube and that solved my problem. All that fuss over a $7 tube.... no more!