Slow leaks occurred at the place where the valve is sealed on the tube itself.

The way it is done is that there is a big round hole in the inner tube and the valve has a round piece of the same material as the inner tube at its foot which is glued or soldered to the inner tube.

The leak appear at this soldering.

Note that due to the very low pressure allowed in the inner tube when not mounted into a tire, its very difficult to locate any slow leak because at such a low pressure the leak is very very small, much smaller than what it is at the normal tire pressure of several bar.