Maybe the installer used the push in "pinch" connection on the outlet and jumped to the light from there? Older wiring doesn't necessarily follow current guidelines or standards. Those push in clamps in the outlets are notorious around our house for failing in odd ways and I wonder if you aren't experiencing a similar failure. I've rewired every outlet in my house using the screw posts for a secure tight fit. Find the proper breaker, turn it off and have a look at the suspect outlet.
