Scotland Yard's little-known Fixated Threat Assessment Centre (FTAC) was set up four years ago to identify and monitor individuals thought to be obsessed with particular VIPs.
The Yard said it was established "in response to evidence that a significant proportion of fixated people are severely and acutely mentally ill and urgently need professional help."
Anyone who's been writing to the Pope, following the Pope, or thinks they are the Pope has had a knock on the door recently and warned to behave themselves.
According to the Home Office FTAC is a counter terror unit and part of the "aim to make the UK a harder target for terrorists by maintaining effective and efficient protective security for public figures."
Its psychiatrists have the power to detain suspects under the Mental Health Act, though civil rights group Liberty fears it poses a threat to human rights.
Senior officers in charge of the Pope's security were a little uncomfortable when I brought up FTAC's role at a briefing, but insisted that no suspect had been locked up ahead of his Holiness's visit.
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