Originally Posted by GypsyR
I don't have the explanation for it but I have noted in testing of certain 12 volt circuits. I recall seeing more than once in service information about how the ground side of a component should show no more than 3 volts positive (or something) when the component is turned off. The instant the component is turned on then that positive voltage disappears and the line goes to full ground. Never made a lot of sense to me but seeing such is certainly not at all unusual.
What you have may be what they call "floating voltage" but I'm not the expert here. I'm sure there's someone more electrically qualified than I around here somewhere to explain.
You are on the right track...This is a link that addresses a very similar circuit
to which the OP has questions. The residual voltage is a byproduct of a pulse forming network that triggers the SCR.
The OP's circuit is typical of early remote switching systems. Later systems have used Single pole, double throw switches that allow better control, similar to home multi-point light control. This is predicated on the OP's supplied diagram being complete. Manufacturer functional diagrams are rarely factually accurate.
The lamps remaining dimly lit is probably a normal condition for the circuit.