This technique has nothing to do with "removing" hum from recordings - it's all about using the very slight - but trackable - variations in mains frequency (50Hz or 60Hz depending on where you are) - to authenticate recordings.
Localised recordings of mains AC frequency can be spectrally tracked so that the variations in the AC frequency can be compared with those found in other recordings such as those captured by the police during investigations. If discontinuities are found in the recordings then it can be a strong indication that editing has occurred within the clandestine recordings.
It does require that a fully tracked recording of localised mains frequency is available for reference.
This is a tried and tested forensic audio technique.