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.