I was asked to install two wall sconces. The old ones were removed but there was only one feed wire (hot from the switch). This is an old craftsman house in california,
I think you answered your own question if there is only one wire to the fixture then the metal box must be completing the circuit through a ground wire attached to the box possibly fastened under the internal clamp. Highly illegal according to any version of the code in modern times.
It is impossible to have a circuit without a completed path. Either through a second conductor or some conductive material such as the earth.
Possibly you found only 1 wire but the other is still around. Knob/tube wiring split the conductors, usually the hot and neutral conductors would travel completely different paths to fixtures. You might just not be looking in the right area. In any case, K&T is not code anymore (for many reasons, including because it's really old and insulation will be degraded), so you'll have to replace it.
You probably know this, but you wouldn't have the neutral going to the switch anyways.