"For the last seven years, at the Metropolitan Police forensic lab in south London, audio specialists..."

[tl;dr - Audio recordings can be timestamped against fluctuation in electrical hum. O_O -egg]
"For the last seven years, at the Metropolitan Police forensic lab in south London, audio specialists...": “
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.

This buzz is an annoyance for sound engineers trying to make the highest quality recordings. But for forensic experts, it has turned out to be an invaluable tool in the fight against crime.
”

- BBC News - The hum that helps to fight crime