I have two light switch boxes in my home that contain "mystery wires," one downstairs by the entrance and the other in the upstairs hallway. Initially, they each had a 3-way light switch that didn't seem to operate anything. I replaced other switches sharing both boxes and now, in the upstairs box, have one wire whose function I cannot determine. The wire actually contains hot and neutral lines (old wiring, so no grounding wire).
Is there anything I can do to solve this mystery? I tried to see if either was connected to a switched outlet but to no avail.