I am reworking a few things in my bathroom. the main box in there has a connection that I simply cannot figure out. it is a 1973 construction remodeled right before I bought it in about 1991. when I pulled the massive mirror I found old outlet box and wall fixture properly cut and dead so I dont think the 90 remodel was a hack. But I simply cannot figure out what this powered connection is for. I have disconnected it and gone over every switch, outlet and appliance in the place and nothing isn't working.
Any tips, or techniques from pros and old hands in solving this mystery? (apart from ripping open walls)?
It may be just capped off in a junction box somewhere...there really is no way to tell you from here. It does sound like you have done what you can to figure it out. I would leave it disconnected and live with it. Someday you might run into something that isn't working.
Answers based on the National Electric Code. Always check local amendments.