I have a small, octagonal junction box in my basement with one 2-wire-no-ground cable going in and a standard porcelain lightbulb fixture attached to the bottom. In the process of investigating an unrelated issue on a different circuit, I discovered that my non-contact voltage tester would go off anywhere in the vicinity of this box. I pulled out the multimeter, touched one lead to the box, and the other to the ground socket of an extension cord plugged in elsewhere. It showed about 5V. When I switched the circuit off, that 5V went away. I opened the box up and checked to see if there was any worn insulation or bare wire contacting the metal, but I couldn't find anything that looked fishy.
There are other similar junction boxes down there, but none of them set off the voltage tester and none of them show any voltage on the multimeter. Any idea what could cause this? The number 5 in particular is confusing to me. Contact with the hot wire would give 120V, but how can you get 5? All the components here (box, wires, fixture) are probably about 60 years old.