According to Jeff Wisnia :
An inbalance in the current flowing in the black and white wires is not
*all* they detect Doug.

Actually, it is all they detect.

Most folks who understand that much about GFCIs don't know that nearly
all GFCIs will also detect a neutral to ground fault downstream from them.

How do they do that?

Because the current flowing in the black and white wires isn't balanced,
some of the current that should be going thru the neutral is going down
the ground wire. Even if _nothing_ is turned on in the circuit in question,
there'll often be enough current flow (due to slight differences in voltage
between neutral and ground) to trip it - an implicit ground loop.
--
Chris Lewis, Una confibula non set est
It's not just anyone who gets a Starship Cruiser class named after them.