He looks more than a little shifty. Unshaven, he is wearing a red hooded top, with the hood pulled over his head despite the heatwave outside, dirty faded jeans and gleaming white trainers. He is also glancing about suspiciously, reeks of alcohol and is clearly having trouble standing up.
As always, those desperate to blend in stand out a mile, even in the packed ticket hall of Old Street Tube station.
He walks on, but just as he prepares to enter the Underground he stops dead in his tracks and tries to return the way he came. He has spotted one of the new, highly publicised mobile metal detectors employed at Tube stations - not to counter terrorism but to address the surge in knife crime that is sweeping the capital.
Recent reports suggest that knives are becoming almost a fashion accessory among young men. Even schoolboys are carrying them as a matter of routine, albeit ostensibly for protection in the playground. One 15-year-old, a football prodigy with Queen's Park Rangers club in west London, was stabbed to death in a school fight recently.
At Old Street, our suspect, aged about 25, is stopped as he backtracks through the ticket hallway, accosted by three British Transport Police officers, and manhandled to the side of the hall. A plain-clothes officer, dressed in the requisite 'undercover' baseball cap and tan leather jacket, motions for the man to empty his pockets.
He turns out what looks like keys, coins, notes and cigarettes. His back pockets reveal something more sinister - a 15cm-long metal object which doubles in size when the policeman flicks a switch on its side. Caught red-handed with a flick knife, the man is arrested, handcuffed and led away to British Transport Police holding cells.
Our suspect is but one of scores of nefarious characters caught out by the new scanners. In the first month of Operation Shield alone, 10,000 travellers were scanned at stations where knife crime has become a serious problem, with 100 arrested. Sixty-eight of those arrests were for carrying knives, the rest for carrying stolen goods or for drug offences.
While the scanners cannot detect drugs, they act as effective deterrents, prompting suspicious characters to stop and shy away. That alerts plain-clothes 'spotters' and police watching on CCTV, who then tip off uniformed officers.
The scanners, which can be moved with ease to different robbery hotspots, have been so successful at lowering knife crime and robberies on the Tube that they will soon be employed at other major stations around the country, including Manchester, Bristol, Glasgow and Cardiff.