No problem. Glad I could help you narrow it down.
I want to correct this statement you made to John: "where there is voltage there is current"
It is possible to have voltage without current flow. However, you cannot have current flow without voltage. Voltage means there is the "potential" to have current flow. Think of a battery. If you measure the voltage across the positive and negative terminals with a multimeter, you will see voltage between those two points. Therefore, there is the "potential" to have current flow between them. However, there is no current flow because the circuit is not complete. If you were to put a wire between the terminals, you would complete the circuit and then you will have current flowing in the wire (don't try this part with a 100aH lead acid battery!)
Therefore, if you measure the voltage between the 24v house bank (+) and the Yellow/Green bonding wire you will see a voltage reading that is very close to your house bank voltage (24-28v). If you measure voltage between the 24v house bank (-) and the Yellow/Green bonding wire with a really good multimeter you might also see a very small voltage potential (probably millivolts or zero).
Here's my understanding of how the "Masse" leak detector works. The Masse + and Masse - tester puts an LED light bulb in a switched circuit between 24v (+) or 24v (-) and the Yellow/Green bonding wire. When you close the switch it completes half the circuit. If there are no connections between 24v (+) or 24v (-) and the Yellow/Green bonding wire, nothing happens. However, if somewhere on the boat you have a connection between 24v (+) or 24v (-) and the Yellow/Green bonding wire that completes the OTHER half of the circuit. Therefore, current will flow and the LED light bulb will light up. The more current flowing, the brighter the light.
I periodically "test the tester" by purposely connecting 24v (+) or 24v (-) (one at a time) to bonding and then switching the Masse + and Masse - to make sure the LED light lights up. I do this behind the Nav station "curtain" as there is relatively easy access to 24v (+), 24v (-) and the Yellow/Green bonding wire. Usually I do it right after I read a thread like this one! The test has always worked and thankfully, I have never found a leak.
Hope this helps!
SM #440 Cara
Seal Bay, ME USA