If you're talking about phantom voltages:
They come from several sources and they're real. The voltage your digital multi meter (DMM) shows is really there. but the power available is so low that that once any sort of tiny load is put on the wire, the voltage drops to nothing.
Even a piece of wire with the ends attached to nothing can have a voltage difference between the ends, especially if it's next to an energized wire. The energized wire's magnetic field "induces" a voltage in the unconnected wire. If you put your DMM on both ends of ths wire, you'll get a voltage reading. But if you use an analog meter you'll see no voltage because an analog meter needs a little bit of current to move the needle and there's no real current available. DMMs are great but this is one of their down sides. DMM's have infinate internal resistance (or almost) and don't bleed off phantom voltages. If you're not sure if the voltage is real, use a wiggy type meter to test.
naysayer, skeptic, conspiracy theorist