I had a similar kind of problem, and the leak was difficult to locate even when the tube was immersed. I had to pump more air in the tube and "flex" it a bit to get the leak visible. It was on the outside, suggesting that something poked through my outer tyre.
So I checked the corresponding spot from my outer tyre. There was no visible damage on the inside surface and I could not feel anything poking through. On the outside was a small hole, and in the bottom of that hole sat even smaller fragment of glass.
Apparently when the tyre was fully inflated the piece of glass would work its way through the tyre, puncturing the tube. When tube was deflated and removed, the glass would be entirely inside the tyre, making it very difficult to notice.
Good luck with the exercise and be patient. If everything else fails, replace the outer tyre (if the puncture is on that side of the tube, that is).
--J