What's the best way to track buried electrical wire that's not live?
There are basically two types of wire tracer:


audible frequency transmitter and receiver often used with telephone wires to detect them in floors, walls, etc.  It is expected the receiver can be placed within a few inches of where the traced wire potentially is.
low frequency RF (400&ndash;500 kHz) intended for detecting buried wires or inside structures where the receiver detector can be up to a few feet away.
There are some which can do both audio and RF.


Each type has its tradeoffs.  The RF system can be difficult because of how the trace signal propagates through conductive conduit or nearby wires (like grounds) so it often seems like the "wire" is everywhere.  The audio detector often seem like the signal disappears and it is only by occasional blind luck that you find the wire.

In the case of a wire buried in a yard, I would think the best way is to dig up the wire.  That way you can easily follow it and get a great feel for how its installation is aging, how well it is staying out of the way, etc.