I had the same problem- very slow leak. I tried EVERYTHING, then
replaced the toilet and the new one was fine. It must have been a very
small crack in the old toilet. I would just go get a new one if I were
you.

Eric in North TX wrote:
I had one that the internal glazing was damaged, and it wicked water. I
just covered the area with a thin layer of silicone seal, that was over
5 years ago, and it hasn't leaked a drop. I find that will even seal
cracks, seems like the ideal material to seal with silicone seal, as it
makes a long term repair.