Method and system for detecting an indicating device

A method of detecting the presence of an indicating device in a restricted investigation zone, comprising the detecting of an indicating device (31) of a highly permeable material by emitting and receiving a magnetic field, where two coils (1, 2) are caused to emit a high-frequency magnetic alternating field, and a first one of the coils (1) emits an alternating field with a frequency (f,) different from the fre- quence (f₂) of the field emitted by the second one of the coils (2), and at least one difference and/or sum frequency n f₁ + m f. occurring by intermodulation by means of the indicating device (31), where n and m are positive or negative integers, is caused to be received by one or several coils (9). According to the invention, a third low-frequency magnetic alternating field is caused to be emitted in said zone with a frequency (f,) substantially lower than the frequencies (f₁, f₂), which low-frequency field is caused to have a field intensity sufficiently high, independently of the fields with the frequencies (f,) and (f₂) to put the indicating device (31) into saturated condition in step with the frequency of the low-frequency field. A filed re-emitted from the indicating device (31) hereby is detected by means of a detector device as an intermodulation occurring periodically with the frequency of the low-frequency field between the frequencies (f,) and (f₂).