Alright so my back left tire loses pressure extremely slowly, and by that I mean 1-2 psi a week, maybe. If I drive it every day, it loses less air than if I let it sit. I have dunked the wheel, no bubbles, the rim is flawless, there is no puncture in the tire that is visible to the human eye.

What do I do? Again, I have already taken the wheel off and inspected it (well), dunked it in soapy water, etc etc. Please advise.