I'm sorry Paul, but that sounds rather like the sort of urban myth masquerading as fact that one would expect to read in the Daily Mail or the Express.
Prosecutions are brought by the CPS, not the police, and they do actually need some evidence. It's not like the case of the over-zealous photo lab calling the police when pictures of naked kids are found, as has happened.
Weelittledrummer has given the best advice.