We investigate: surveillance technology

It is possible to obtain a pay-as-you-go Oyster card that can still be tracked but is not linked to your identity. Buying a train ticket also leaves electronic footprints and CCTV cameras in stations spot even those paying in cash.

On trains across the country, you can see cameras in operation when you pass the train manager’s office on a Virgin Pendolino service, observing each carriage. On buses too, small, shiny, black camera domes record what goes on.

Once in a shopping centre, CCTV cameras operated by individual stores and by the centre itself, in conjunction with the police, keep an eye out for shoplifters. But private companies that are not connected directly with the centre may also be monitoring your movements while you shop, sometimes in novel ways.

One such system is Footpath, run by Path Intelligence of Portsmouth in Hampshire. The system works by detecting the signals of mobile phones and is already installed in several shopping centres in the south of England, including the Gunwharf Quays centre in Portsmouth.

However, even this system, which sounds slightly sinister at first, is strictly controlled and anonymous, as Path Intelligence is at pains to explain. “When we developed Footpath we contacted a number of organisations including Liberty, the Electronic Frontier Foundation and the Information Commissioner’s Office.

"Acting on their feedback we introduced important changes to our system to make it more secure. We are continually looking to engage with critics and improve our practices,” a company spokesman said.

But isn’t it possible that the collected data could identify individuals? “We have no idea who you are as an individual,” said Path Intelligence. “We only look at the path your phone takes­ – it’s like looking at a dot moving around. In isolation this information isn’t too interesting until you look at the wider trends and see many dots taking the same route or visiting the same areas.”

By noting when and where these dots collect, security staff can be directed to places where pickpockets are most likely to strike, for instance. By discovering which areas of a shopping centre are more popular, cleaning staff can be better deployed, too.

One criticism of this type of system is that, if hacked, individuals could be identified without their knowledge or consent. As a result all phone IDs are automatically scrambled before use.