I found the infamous Dining Room Circuit To Nowhere. And it appears Smoking Too Much Reefer Contractor is the culprit.
Here it is, in the unholy mess of a junction box:
See that top wire labeled with a "14"? I suppose I should thank him for labeling it. So nice of him. Maybe next time he can ACTUALLY CONNECT IT TO SOMETHING.
For now I have shut off the breaker. No point in having electricity running to it if it's not going anywhere. And I actually gave him the benefit of the doubt.