Hi Zeus,
In my experience most ground fault are caused by:
1) Installation; wires pinched in a back box or a back box connector screwed in too tightly.
2) A supervision switch on a sprinkler system that has gotten filled with water after a defective seal.
3) Quite often I get the call after a painter had removed a piezo in a dwelling unit and pinched the wires upon reinstalling it.
Also water infiltration in heat or smoke detector is a common thing as well
To trace your ground fault, the methods described by Frank in the above post are your best way to go, but I also found a way of doing it sometimes faster if I'm lucky.
I use a meter called SureShot made by Platinum Tools which enables me to trace the distance of an open or a short on a cable.
The meter's price is hefty ( In the $300 and above range ) but it is well worth it.
On a class A circuit you would put your meter on both sides and the difference between the two would give you a pretty good appreciation of the location of your ground fault.
My 2 cents