Well, while cleaning up the mess I decided to go ahead and remove the rest of the 2x4 framing since it was so poorly attached to the wall and needed moved anyway.

Once I got it off the wall I found that there is indeed an old power line, live no less, which is at the top of the fireplace.

I'll be using a shallow electrical box to enclose it and will frame a power outlet into the new mantel. Nice to find it, makes my life easier since I don't have to run power to the location.