When a marginally knowledgable person stumbles on to a electrical mystery, with a box that will be jammed when they are done adding a outlet to a already full box the easiest thing.
Forget about the mystery, since at best it will be overfull......
Its far easier to add a box close by fed from a convenient location.
Boxes and wire are pretty cheap. Far cheaper than calling a electrician.
I have some friends who were tough to deal with. They re arranged their kitchen and tried to insist on moving outlets to save $$$.
I said if you want me to do this job and remember I am FREE, then you are buying the necessary supplies!
They were unhappy but not stupid. They papered over one blank box to save the cost of a blank plate, if I had known I would of given them one.......
Espically when dealing with old work running a new wire and outlet is often the easiest solution.
My moms house had a dead outlet:( I checked it out and somehow the black wire right where it entered the box had broke off.... With only a tiny bit showing I wire nutted it and put a blank plate over the now empty box.
Then installed a nice new box and outlet a foot away. Not only was it more convenient, but a far easier job that I knew would last forever since the home was being sold...
Incidently I did lots of work the home inspectors passed everything:)
Except the first buyers inspector flagged the lack of GFCI on the garage sump pump. The deal fell thru for other reasons....... But I added the GFCI.
Home buyer number 2 had his inspection, and flagged the electrical for ONE thing, claimed the same garage sump pump shouldnt be on a GFCI:(