In several of the rooms of the house I moved into last year (built 1973, USA), there are blank wall plates. I've removed a few to investigate and found somehting similar to the below pictures each time: a loop of what appears to be thick brown electrical cable, accompanied by a foot or so of thin 2-conductor black cable that is either insulated with rubber or a very flexible plastic, and covered in orange muck.
I've given the large loop a god tug and it doesn't appear to be an off-cut tucked back up into the wall. If it is, it's jammed in there pretty good.
Neither appears to carry any voltage according to a NCV tester, and it doesn't resemble the mains wiring in any of the live outlets or switches (which is all white plastic NMC and generally a bit thinner than the thick brown wire here).
I'm not intending to do anything with it at this point, just curious as to what it is/was.