A rape victim has come forward to the police. She says she has confronted her attacker and has secretly recorded him admitting his guilt.
A suspected terrorist has been taped planning a deadly attack, and the police want to use this evidence in court. Or someone has been captured on CCTV threatening an assault.
For the last seven years, at the Metropolitan Police forensic lab in south London, audio specialists have been continuously recording the sound of mains electricity.
It is an all pervasive hum that we normally cannot hear. But boost it a little, and a metallic and not very pleasant buzz fills the air.
"The power is sent out over the national grid to factories, shops and of course our homes. Normally this frequency, known as the mains frequency, is about 50Hz," explains Dr Alan Cooper, a senior digital forensic practitioner at the Met Police.
Any digital recording made anywhere near an electrical power source, be it plug socket, light or pylon, will pick up this noise and it will be embedded throughout the audio.