When we had an outbuilding built, the electrician put in a box with a run from the breaker panel and another empty (i.e. completely disconnected) run from the box up the wall because we didn't know if we were going to want a single light over a medicine cabinet or lights along the side. He said he left the other end in the wall somewhere. Now, 15 years later, I don't remember where he put the wire. Is there a way to find it without tearing out the drywall?
Pull out the two-gang box. Should be just a couple of nails or screws holding it to a stud. Fish around in the wall cavity for the cable. Assuming the box was installed at the same time as the cable was run, it should be in the same stud bay.
As far as wire finders and other relatively unusual tools that you don't want to buy for a one-time use (which I totally understand), that's what neighborhood email lists are for. Even pandemic issues shouldn't affect that - you can arrange a not in-person dropoff/pickup.