Even more evil technique criminals could use: carry a device which emits its own fake, randomly fluctuating "mains" sound, thus tainting any legitimate recordings so that forensics analysis will incorrectly identify them as "fake".
Similar to having a bag with lots of different people's hair and skin cells to dust over a crime scene.
Worse though. A bag of hair and skin cells could throw police off your trail. A tainted recording might could make it as far as the courtroom, only to have the defense bring in an expert witness to discredit it.