Phantom voltage -- I'm completely stumped
I live in an older home with a mix of knob-and-tube, older NM, and new NM. In my attic, I have an old subpanel with a bonded neutral (it was originally wired with 8/2 and I need to upgrade it to 8/3). I have a hunch that it was originally the location of a non-grounded fuse box and the previous HO just replaced it with a subpanel but didn't run new wire. There is an old run of K-and-T that powers an attic fan and it also powers the 2nd floor lights (below attic floor).
The subpanel powers a few receptacles and lights in the attic, as well as a few receptacles on the second floor.
Here's what is happening...
When I turn off the breakers on the subpanel (and at the main service panel), my non-contact voltage meter will pick up voltage on long runs of wire and at the "dead" receptacles. I tried this with two different brands of non-contact meters (Klein and Greenlee). My first thought was the non-grounded subpanel somehow had a hot neutral and was energizing the circuits this way.
My first test was with one of the 3-prong outlet testers. It showed nothing.
My second test was with my multimeter and it showed no voltage.
I then tried disconnecting a circuit completely from the subpanel and at the first junction box. This wire run is absolutely 100% not connected to anything. It still triggers the non-contact meter. I can visually inspect the entire run and there is no way that is in contact with any live wires (or any wires at all).
I don't get it and have no idea why two different non-contact meters go crazy in my attic. I really don't understand why a non-contact meter will show voltage but my multimeter doesn't.
Anyone have any ideas?