not that computer savvy boots,Why not construct a circuit diagram for us?
2 wires for expansion, running an unknown distance buried under the mainline
1 wire connected to a valve
screwdriver with same gauge wire as system used as a lead to the testing area serving as an earth ground.
the black lead touching the earth ground and the red lead touching the common wire = continuity tone
i've done many resistance tests in my life and have never gotten continuity from a wire that is not connected in a complete circuit or a loop
i may be confusing in my description, but i think not
i know what i'm reading, but do not have the answer why?