What is this extra wire in my light fixture box?
I wanted to follow up with the correct answer for anyone who might find this thread in the future. We're in the middle of a kitchen/dining remodel, so I waited until the electricians were here for their rough-in and asked them about this hall fixture. One little detail I forgot to mention regarding the two wires from the pipe: The pipe is &quot;filled&quot; with some kind of black solid or foam. Turns out the two wires from the pipe should have been twisted together, but there was no wire nut for them, and they were not making a good connection. This made our DOORBELL not work. It has not worked for the 7 years we've lived at this house. Directly above the ceiling box (in the attic) was the transformer for the doorbell. I've looked for it a bit in the past so I could repair/replace it, but was never able to find it. Also, the electrician noted that the hall light circuit was sharing a neutral with a kitchen circuit. They said that was not ideal, and fixed it.
So in summary, in addition to its normal ceiling box duties, this box was doubling as a junction box for the doorbell transformer AND doubling as a junction box for a kitchen circuit. Mystery solved. Thank you everyone that contributed!