Got a new trainer tire, so I removed the regular tire and tube from the rear rim and replaced them with the special trainer tire and a thinner tube. When I plied the old deflated tube for storing, I noticed that it was leaking air. On inspection I've found 3 pairs of holes which are apparently on the inside of the tube, toward the rim. I inspected the rim tape and it looks fine. In the past I've had put some extra electrical tape on the rim and then the rim tape over it. They both look fine, the electrical tape is not damaged or cut where the spoke wells are. The rims are double walled. I inspected the rims, spoke wells, rim tape, the tire - can't find the cause of the punctures. They seem to align with the spoke wells, though. What makes it weird is that all three punctures are double, there's one cut and next to it there's another, not quite parallel. It looks something like this: / \ . Check the pics also. With the tube deflated one of the puncture has the cuts 8mm apart, other has the cuts 4mm apart and the last has the cuts 2mm apart. What baffles me is that the wheel was not losing air before I took it apart. It was inflated! Only after I removed the tube I've seen these holes which are big enough that I can't inflate the tube now with a hand pump.
What can cause this? It's got to be the rim, right? Somehow the rim tape slides about and exposes the spoke wells. But why it looks fine and why did it stayed inflated until taking it off?
Wheel 26", tire 57-559, tube 40/60-559.