Breaker finder or wire tracer
There are devices sold at different places that are designed for this...sort of...
One is a "breaker finder" that you plug in an outlet to locate its breaker, then other one is used for locating a specific pair in telcom wiring.
They were expensive in days past, now not so much.
They work by transmitting a signal on the wire and have a receiver that is used to locate the wire.
Check the box stores and maybe the local Graybar or other data wiring support stores, you need to confirm the device will work with a single wire in your application.
From there, connect the transmitter to the end you can find and happy hunting.
Otherwise you are upto a bunch of crawling, hunting and busted knuckles.
You may also be stuck with running new wires, get an inspection camera, they are not too expensive and with these you can look into tiny places and retrieve fish wires and other things, we used one to run new plumbing in ours.