I've had a couple of spells like that, and on both occasions it turned out to be a piece of glass buried deep into outer tyre. When inflated it would press against the tube, causing a slow leak. When I took off the outer tyre I could not find the hole on the inside surface. I could spot the hole on the outside, and after looking very carefully in there, locate a small shrapnel of glass. Last time I had to use a bright LED light to find it, and some small pliers to get it out.

I suggest you take one last look at that tyre, and if you still cannot find the reason, just replace it to cut your losses.

--J