"...and now, the rest of the story..."
This is the same dwelling as the one in these threads: Link 1 Link 2
This was found, as well as three flying and hidden splices, and other loveliness. Still no fault found. The AFCI trips when load is applied, and taking the circuit apart piece by piece today had narrowed it down to a splice I can't reach without liberal use of a Sawzall.
So... We'll tackle it again tomorrow...
Oh, this receptacle and switch is/was in a bathroom. Yes, a bathroom. The most abused receptacle in a house. My belief is that it must've never been energized, or atleast never loaded. The "feeder" was snipped off in the wall. I tore boxes and all out and threw them in the trash.
All in a days work.