Someone I once knew - not me, of course - once owned one of those natty little AOR scanners. This thing would pick up most transmissions from DC to light - Or so I understand.
Once upon a time he happened to switch it on in London, apparently and scanned some NFM frequencies somewhere below 1000 MHz. Purely out of curiosity to see what his radio would pick up, of course... Allegedly. Some minutes later he was somewhat startled to hear a voice say "Listen, you slag - I know you can hear me. This is PC *** and I'm telling you that either you give me the £500 or you'll get nicked...."
Now came the problem. Should he ring the Met and tell them what he'd heard and risk prosecution for eavesdropping? Or keep schtum and allow some bent copper to get away with criminal activity.....
I understand that he took a third option. He sent an anonymous letter to PC*** advising him that his call had been recorded and that copies were being forwarded to the authorities.... Or so he said. Allegedly....