Is there an instrument that can measure electronically the distance from a point in space through something like bone to a very low voltage wire?
Can you control the voltage? If so, you could measure it capacitively or inductively. Inductively would be to put an AC current on the wire and use a pickup coil as your measurement device. In general, measurements of this kind are greatly aided by an AC measurement.
Of course, if your wire is flowing current (AC or DC), there are innumerable fancy magnetic field detectors.