Brian is back with his squeaking floor -- see Just Ask Jon Eakes, Season 3 Episode 15 for the original problem.
I had asked him to use a broom stick as a stethoscope to get a precise location on the squeak. He did me one better and used his kid's toy stethoscope, which worked wonderfully. The squeak was definitely coming from the wall to floor junction right in the middle of the stairwell span. Then he pulled back the rug and saw a crack under the wall.
Brian wanted to screw the wall down to the floor, but I was worried that might just pull the wall off of the ceiling above. Rather I suggested that he load the hallway floor with weights to push the floor down, and then inject PL Premium construction adhesive under the wall to fill the gap. PL Premium dries rock hard, and should lock it all together. When the weights come off the floor (after the glue is hard) the floor will not have a tendency to go down when you walk on it because it has been pre-loaded and forced to stay in that position. Remember, we are talking about 1 mm of movement, about the thickness of a pencil line.
Brian says it worked perfectly, and his kid now sleeps all night long. I asked Brian to let me know if it turns out to be a temporary fix, or is it permanent.