Capacitive coupling from a hot wire into an ungrounded wire will give exactly the results the OP mentioned, If there is a third wire associated with the first two wires, and that wire is ungrounded at either end, it will also pick up a signal triggering a sensitive voltage tester. But, if that third wire is grounded at either end, there should be no voltage induced/capacitively in the ungrounded second wire. If the third wire is gounded at the near end, nothing should be measured. If the third wire is grounded at the far end and at the near end, and carries some return current, there can be magnetic coupling to the third wire and some voltage may be measured.