This is happening to me in a US residence. I’m replacing two light fixtures and both switches. One light has a 3-way switch, the other just a single-pole switch. See crude wiring diagram.
For the following observations, I’ve disconnected all black, white and red wires in both boxes and removed the lightbulbs from the fixtures. There isn’t anything else on either switch than the one light on each but I even removed the switches for these tests.
I turned the breaker back on to carefully investigate. Using a DMM, if I measure the voltage between the hot (coming from the panel) and any other wire in the double gang box except those to the single pole switch, I get about 105VAC. I can understand how there’d be 105VAC between it and the neutral but how can there 105VAC to the other wires that should be feeding the other three-way switch and ultimately the light? I assume the neutrals to the other 3-way switch and light fixture would be about 0VAC since presumably these neutrals wouldn’t be tied anywhere else.
There’s no voltage between any of the wires in the single gang box. Also, I’ve measured the resistance between all the wires in the single gang box and they’re all infinite ohms. Even the blacks, whites and reds are infinite ohms to the ground on both ends. Since there’s infinite ohms, this suggests there’s no short like a screw through a wire in the wall, right?
Still – how can there be 105VAC between the hot and any other wire going to the other three-way switch?