Even if other energized conductors are not actually carrying current to a load, you will still get a reading from the capacitive coupling. Two conductors running parallel separated by just their insulation form a very effective capacitor. The longer the cable run, the higher the capacitance and the higher the voltage reading.
You'll see the same capacitive effect if you use an older-style analog megger for insulation testing. When you press the button, the needle will "kick" over to the right, then gradually drop back to the left. The longer the cable, the greater the "kick" and the longer it will take to drop back to its steady value. The D.C. you are applying to test is charging the capacitance of the cable.
