Well, my favorite expression is, "All the world's a voltage divider." All the wires and conduits are resistors. The lower the voltage, the closer you should be to the contact point with earth. You might try using a clamp-on ammeter on the conduits to see if you pick up fault current.
The last time I had to track down a ground fault, it was an FACP signal circuit. The culprit zone was the one that measured the closest to ground. Lifting the wire shifted the fault to the other rail, through the faulted device.
In your case, you should probably verify that your ground detector isn't the source of your ground fault: Snubbers, MOVs, gas discharge tubes, Et,c.