Slow leak that fixed itself? I noticed that my back tyre was losing pressure -- on the order of 70 psi down to under 30 psi in three days. (I pumped it up and left the bike alone for those three days.) I thought it had a small puncture, so I took the tube off and examined it very carefully, dipping it in water to find any leaks. I did not find any.

After remounting the tube and filling it up, it is currently (after four days) holding the pressure well.

Is this something to be worried about? Temporary valve malfunction or something else?

EDIT: Now, one and a half weeks since the last pumping-up, the tube had indeed again lost much of its pressure, though the pressure loss was quite a bit slower this time. I am betting the cause is a badly applied patch. (How can I find which one it is?) Should consider just replacing the tube.
In the end, it seems I had a badly applied patch that was leaking.

I dipped the tube in water (again) and there were small air bubbles coming from the edge of the patch, maybe one per second. I tore off the patch and installed a new one.

It leaked more when I rode the bike, likely due to the extra weight. When I wasn't riding it, the air loss was much slower which lead me to think that it had fixed itself. Also, my pressure checks were inaccurate, done with fingers only, which can't really tell the difference between 40 PSI (the minimum) and 75.