I had a strange puncture in a MTB tube.

The wheel was getting partially deflated overnight or after a 15-20 mile long ride, but could still be ridden.
I removed the tube and tried to find the culprit by immersion in a water container but could not find any so I replaced the Schrader valve core assuming a failed valve core and reinstalled the tube. No improvement.

So, I tested again but this time squeezed the inflated tube, too. Air bubbled out from a very small puncture right next to the valve.

I have repaired it but I am wondering if this is a freak thing or is the tube which is a few years old, is ready for replacement.

Check the valve hole in the rim to be sure there isn't a burr or if the rim tape has moved exposing the edge of the hole.

I inspected the rim and while the rim itself was not causing the puncture, it was the rim strip because the rim strip on that rim is made from a hard plastic tape( strapping tape I guess) and the hole for the valve was burned in the tape and had a sharp raised edge. I tried to file the sharp edge off but it was not working so I made a wide (1.5" square) washer from a piece of an inner tube and put it on the valve to prevent direct contact with the sharp edge. I had seen this done by pro bike mechanics in India where I grew up and did not give a thought to why they did it. I am almost certain that should take care of it. Thx HillRider.