I recently moved into a home that's about 20 years old and have a problem with the 15 amp outlet circuit in the garage. I was using a 12 amp table saw on the third and last outlet along the wall when the GFCI (the first outlet in the series) tripped. I had powered the saw up several times without any trouble, but one time the GFCI tripped right after I powered up, then would not reset.
Turning off the breaker and checking everything, I found that the GFCI outlet and the two ordinary outlets in the series were good, but the third (end) outlet had wires that continued downstream from it to unknown points. When I disconnected the downstream circuit from the third outlet, the GFCI reset successfully and all outlets worked fine.
As a temporary measure, I terminated the downstream circuit with wire nuts and electrical tape, so whatever it's for, it's now a dead circuit.
Though there are a couple other outlets on the other side of the garage and a garage door opener outlet on the ceiling and a garage door opener button switch all connected to the same GFCI, everything is still working correctly. I cannot find any outlet or device powered by the circuit downstream of the outlet I plugged the tool into.
My questions are 1) Any idea what caused the fault? I suppose there was a startup current spike that damaged something downstream before the GFCI tripped. 2) Any tricks or other ideas how I can find where the circuit goes and what it's for? It goes upward toward the ceiling from the outlet. I don't see any junction box or device powered by the circuit. Directly above the garage at that point is the living room, and every outlet/light is fine up there.
EDIT 1: Following Rab's advice in the accepted answer, I tested for continuity with the multimeter between hot/ground and neutral/ground in the downstream circuit that trips the GFCI when connected. Hot/ground are not connected, but neutral/ground do show connectivity with a resistance of about 40 ohms. So it seems to be a permanent neutral-to-ground fault at some unknown point. I will follow Rab's advice on tracing the circuit but have to buy a new NCV detector as suggested!