A method, device, and apparatus for tracing a conductive line and locating any concealed surveillance devices coupled to the line uses a signal generator to produce a test signal having a fundamental frequency which is coupled to the line under test. The test signal flowing through the line under test creates electromagnetic waves that propagate through the atmosphere away from the line. A portable locator probe is used to detect the radiated signal and thus the conductive line by detecting the magnitude of the radiated signal. As the locator probe is moved closer to the line, the amplitude of the detected signal increases. In addition, the portable locator probe detects harmonic signals radiated from nonlinear junctions coupled to the line at harmonic frequencies of the fundamental test signal. By examining the relative strengths of the second and third harmonic signals, a user can determine if the detected non-linear junction is being produced by a semiconductor or a corrosive/dissimilar metal type non-linear junction. A DC bias voltage can be used to improve the responses of any semiconductor based non-linear junctions. Any semiconductor junctions located with the probe are manually examined to determine the cause of the non-linear junction.