As a countermeasure against an NLJD, professional covert listening devices (bugs) of the Central Intelligence Agency were equipped from 1968 onwards with a so-called isolator. An isolator is a 3-port circulator of which the return port is terminated with a resistor. Any energy injected into the bug by an NLJD will be absorbed by the resistor, resulting in no (or very little) reflected energy. An example of such a bug is the CIA's SRT-107.
(or my favorite:)
A means to hinder isolating a non linear junction is to add inexpensive diodes to places where the NLJD is expected to sweep. This masks the true listening device against a field of false alerts when the many diodes are detected. Such a technique was used in the 1980s construction of the U.S. embassy in Moscow. Thousands of diodes were mixed by the Soviets into the building's structural concrete, making detection and removal of the true listening devices by its American occupants nearly impossible.
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Plus if the electronics aren't shielded you can use a non-linear junction detector even if it's turned off.
Ah, unless your spy is clever....
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Rusty nails can also be a false positive. Nothing is perfect but you always have to evaluate your threat profile and mitigate accordingly.
The later scanners filtered out such false positives with extended harmonic analysis.