Before drilling a wall (drywall at the surface; not sure what's beneath) separating the bathroom from the corridor, I took a multimeter with a non-contact voltage (NCV) detector to check that the location where I want to drill doesn't have wires underneath.
To my surprise, the NCV beeped for… the entire wall. (Note: for other walls, the NCV beeps only in locations where there are wires inside, i.e. above the outlets or switches up to the ceiling, or sometimes to the left or right of them.)
If I cut the power for the whole apartment, NCV stops beeping. It also stops beeping when I turn off at the service panel just the items labeled “bathroom” and “lights,” however, if I turn off just one of the two, beeping continues.
What could be a possible explanation for that? What could it mean in terms of safety issues?
Note: there was a practically identical question asked before, but the answer which was provided doesn't apply in my case. The multimeter is quite new (bought a few months ago), the batteries should be quite new as well (haven't used the multimeter that much since I bought it).
Comments are not helpful either: I doubt there are metal profiles in the wall, given the location of the wall and how thin it is. If there is water, this could be, I suppose, a troublesome thing. The detector is indeed triggered when in direct contact with the skin, but not within a few millimeters. When I check the wall, I keep the multimeter at a few millimeters as well.