I've already seen it happen at least 4 times in my back tyre and once in the front one.
After installation of the new tube, after some time of cycling (days) the tube will start deflating. Most of the cases it was very slow so that I needed to pump it every 4-5 days. The latest is much faster.
I checked some of the tubes and couldn't spot any hole, even when submerging in water.
I have always cleaned and checked the tyre itself and couldn't find any debris that could be re-puncturing the tubes. Same issue with winter tyres. I have used both for a total of 7000km, they look in shape tho, the patterns are still very visible.
Still using the same brand/type of inner tube, they worked fine until a year ago when this issue had started.
What could it be?