Traveller Wire Showing 40V When There Should Be None
Are you by any chance using lighted switches? If so they are likely the source of the voltage you are seeing. The 3-way switch is a "single throw double pole" switch that has a resister and light (usually neon) between the poles. This allows a small amount of current (milliamps) to flow to the load even when the switch(s) are off in order to light the switch. 40 volts sounds about typical for an open load i.e. no light installed but there is not enough current to light the light. This is true for incandescent lights, LED lights could light dimly or flicker.