This may sound a bit crazy , but perhaps it is also why no one can find youre leak.
Check your gutter on your roof if you have a seperate flashing running fron your gutter to under your shingles your probably still looking for that leak , but in my 38yrs experience with rfg, and Home Inspecting I have found the lack of this flashing may allow the water which runs over the shingle into the gutter , by means of capillary action to run down the inside of your walls and generally come out in some obscure place in your basement.