Wow, this is some seriously impressive forensic work. Steve is going to dig this… :-)

Three men were accused of selling firearms to South London gangs. At their 2012 trial in Croydon Crown Court, the prosecution played the jury a recording, taken undercover, of the trio allegedly arranging a sale. But the men’s lawyers claimed that the recording was a fake, and that the police had fabricated it by splicing together clips taken at different times. To prove that the evidence really was authentic, the Metropolitan Police turned to a technique called electrical network frequency (ENF) matching.
How to date a recording using background electrical noise | Robert Heaton