How to find the mystery red wire
The only sure way to hunt down missing wires, without taking forever, without disturbing the walls, ceiling, or floor is to use a circuit tracer. I use a Mastech MS6818. It comes with a transmitter and a tracer and can be used on energized and unenergized circuits. It may be worth a homeowner to have one if he owns a house like yours and you are determined to track everything down. You might find a local contractor who also has and uses such a device, but I wouldn't let him in unless he has one in his hand when he comes to the door.

If all of that is too expensive your only other option is to verify that it is not a switch leg or some such and doesn't go anywhere. Then just wire nut it off and leave it in the box, then at least keep a mental note as to where it is in case you find something not working.

Good luck and stay safe.