Best bet would be a combination of voltage detector and metal detector. If the wire has no electric field around it (unpowered) the metal detector will find it. To find low voltages you must cascade a lot of amplifiers to a gain of finally 1.000.000.000:1.
I did a circuit some decades ago indicating voltage walking on a carpet caused by friction. (that circuit used transistors) Use a twin circuit of low noise OpAmps, one for positive going voltage and another one for negative voltages or use dual power supply for one circuit. For detection use antennas as already posted here.