3 phase minimum acceptable low voltage
I suspect that you are measuring noise or induce voltage on the cable while using a digital multimeter with a very high (1 to 10 M&Omega;) input impedance.



Figure 1. A Fluke 117 meter with a LO-Z measurement range.

Interestingly, Fluke have a range of meters with a Lo-Z range to avoid this problem on DC and AC circuits. The input impedance is about 38kΩ (from memory). This is low enough to give a zero reading when measuring voltage on an open circuit even in the presence of 'noise' on adjacent lines. 

In the absence of one of those you could connect a small light bulb or relay coil across the terminals. This will probably load the circuit enough to cause the voltage to collapse.