In case anyone is wondering how this worked out, I posted at an electricians forum and they pointed out that I had a GE AFCI (and yes, the breaker was an AFCI, not a GFCI!) in a Siemens panel. Also, the circuit looked like it has been wired as an MWBC at some point, and then modified to run the two branches off a single breaker. So I replaced the AFCI breaker w/a Siemens (which indicates why it tripped, which will be handy if it happens again!), pulled a new cable so I could have two actual separate circuits, and started checking the wiring at the switches and can lights. That’s a big job at not nearly finished! All the connections look good so far but I did find a few “interesting” things they did when they hooked everything up. I will continue to check all the connections but I’m hoping that maybe the changes I already made will solve the problem. Given how random the trips are I won’t feel confident the circuit is fixed for probably a year!
I’m curious about this statement. My house, built in 2001 was required by code to have AFCI breakers in all bedrooms. All outlets and switches are on these circuits. In 18 years, I’ve only had one trip once and that was because I forgot to close the panel which is outside and it started raining.
According to this, lights are included in the requirement:
https://en.wikipedia.org/wiki/Arc-fault_circuit_interrupter:
In 2002, the NEC removed the word “receptacle”, leaving “outlets”, with the effect that lights and other wired-in devices such as ceiling fans within bedrooms were added to the requirement. The 2005 code made it clearer that all outlets must be protected despite discussion in the code-making panel about excluding bedroom smoke detectors from the requirement. “Outlets” as defined in the NEC includes receptacles, light fixtures and smoke alarms, among other things. Basically, any point where electricity is used to power something is an outlet.