A couple of months ago the rear wheel of my Bike Friday developed a slow leak. I'd pump it up to 4.5 bar, but by the next day it would be down to 3 bar or so. If I didn't ride for a few days I'd find it virtually flat. Meanwhile the front wheel was perfectly fine, it would hold its pressure for weeks, same as for the tires of my Elephant National Forest Explorer.
Eventually I checked the rear tire for a puncture but found nothing obvious. So I installed a freshly purchased tube (Schwalbe SV 7A) and hoped that would be the end of it. It made no difference whatsoever, the exact same pressure loss pattern continued. I questioned my own sanity, had I perhaps reinstalled the same tube I had removed?
This went on for some time, before I finally splurged on yet another fresh tube. When I removed the old one, I noticed that the textile rim tape wasn't completely uniform but looked kind of dimpled at the spoke holes, as if it wasn't tight enough, as if it had gotten squeezed a bit into the spoke holes by the air pressure in the tube. This would have caused the rubber in those spots to stretch somewhat. I decided to add another layer of rim tape, tightly wrapped around the rim bed, before I installed the tube.
The next day there was no pressure loss. Exactly the same air pressure to which I had inflated the rear tire the night before. Problem solved!
Now I have two perfectly good spare tubes. If you find yourself puzzling over a slow leak, don't forget to check your rim tape as a possible source of problems.