A leak really takes allot of investigatory skills and patience. I would not do any damage to the exterior of the house, you could tear the complete home down and never find that leak.
Since its an old leak, I would start in the attic above that area and straight up the fall of the roof. Look for a dark water stain that could actually begin yards from where its exiting. Look on the rafters. Most leaks begin high on the roof and then run down something until it hits an obstruction then drips.