Capacitance can add up surprisingly quickly in a long run of cable.
Even at the relatively low capacitance value of, say, 0.1 microfarad, which is fairly easily achieved on a lengthy run the capacitive reactance is
Xc = 1 / (2 * pi * 60Hz * 0.1uF) = 26,525 ohms.
Compare that with the typical 10 meg input resistance of a digital meter, and you can see why the resulting RC potential divider results in these "phantom" readings. If you loaded that wire by connecting say a light bulb to ground, however, the capacitive reactance would then be much higher than the resistance to ground and the voltage reading would drop to near zero.
