random circuit breaker trips under "no load"

[Venkman]

I say "no load" because nothing is plugged into the only receptacle and all lights are not illuminated. The only light that should be powered is the external motion detector spot light with it's switch being turned "on". All the other lights on the circuit are single or double pole switches.

Background: A few months ago I came home and turned on a light switch with no result. Checked the cb panel and found the a lighting circuit cb tripped. It reset with no issue. A few days/weeks go by and the same thing happens again. This time the cb can't be reset. Waiting a few hours or until next morning allows the cb to be reset. At the present time the cb was able to be reset but after turning on lights on the circuit it popped again (can't definitively tie it to a particular light or switch). Leaving it off.
Each time this has tripped no lights have been turned on and nothing has been plugged into the lone receptacle on the circuit.
I tightened the screws on the cb at the panel, no joy. I swapped the wire to a different 15A cb and that tripped also, so I think the cb is ok.
I checked the wiring and the connections at the only receptacle and that seem good.
I can check all the switches and see if the connections are tight and look for shorts there.
Items on the circuit: Fluorescent ceiling and closet lights, one receptacle, one exterior flood light with a motion detector.
Question: Does having a light switch off isolate the associated light fixture from the circuit for trouble shooting purposes or do I physically have to disconnect the wires at the light fixture to see if that fixture is causing a short?

[adnadeau]

To answer your question, under normal circumstances, a properly wired switch will isolate a light fixture for troubleshooting. But you don't know exactly what you have here. Is any of the wiring aluminum? Do you have a separate switch to shut off the external flood light? Is the circuit breaker hot to the touch when it won't reset? Can you smell anything strange around it or the other elements of the circuit? Has anything been changed, added or subtracted from the circuit recently? How old is the wiring? Is it done with Romex or something else? I have seen an incident where an uninsulated staple was driven in crookedly to secure Romex, and after some years caused a short circuit. Are all of the switches just toggles or are there any dimmers? Why do you have double pole switches in this circuit?