Sara needed some chocolate - she had had one of those days - so wandered into a Home Bargains store. Within less than a minute, I'm approached by a store worker who comes up to me and says, 'You're a thief, you need to leave the store'." Sara ... was wrongly accused after being flagged by a facial-recognition system called Facewatch. She says after her bag was searched she was led out of the shop, and told she was banned from all stores using the technology. Facewatch later wrote to Sara and acknowledged it had made an error. Facewatch is used in numerous stores in the UK. It's not just retailers who are turning to the technology. On the day we were filming, the Metropolitan Police said they made six arrests with the assistance of the tech. 192 arrests have been made so far this year as a result of it. But civil liberty groups are worried that its accuracy is yet to be fully established, and point to cases such as Shaun Thompson's. Mr Thompson, who works for youth-advocacy group Streetfathers, didn't think much of it when he walked by a white van near London Bridge. Within a few seconds, he was approached by police and told he was a wanted man. But it was a case of mistaken identity. "It felt intrusive ... I was treated guilty until proven innocent," he says. Silkie Carlo, director of Big Brother Watch, has filmed the police on numerous facial-recognition deployments. She says that anyone's face who is scanned is effectively part of a digital police line-up.