In this age of identity theft, financial fraud and terrorism, most of us know that we shouldn’t give out our personal details to complete strangers.

Yet many of the high-tech items that we carry with us every day may be doing just that – silently broadcasting our nationality, location and identity to anyone who might want to listen.

Less than £50 buys a system that can locate someone to within yards. Put a network of detectors together, and you can follow one person or a group as they move through a city. Sounds like the plot of a paranoid thriller, such as Enemy of the State? Unfortunately not. The thing that gives away our location is a low-power radio system called Bluetooth. Virtually all modern mobile phones have Bluetooth to let you use hands-free handsets. Laptops and personal organisers use it to swap files, while some sat navs have Bluetooth installed too. There are even MP3 players that come with wireless Bluetooth headphones.

When they set up their Bluetooth identity, many people enter their own name to enable file sharing. Visit www.bluetoothtracking.org today and you can see live data on more than 100,000 people in seven locations in the Netherlands, and even search for individual first names. The site does not publish full Bluetooth phone codes or names, but others may not be so ethical.

The (anonymous) author of the site says: “It’s amazing how many people put their full name or last name in their phone as a Bluetooth identity.”

Even if a person doesn’t use their own name, a determined tracker can discover with a little ingenuity what code they have chosen and track them with a network of Bluetooth receivers.

In researching crowd behaviour, Eamonn O’Neill of Bath University’s department of computer science showed how scanners could be used to target groups of pedestrians. He discovered that about 7.5% of pedestrians in Bath were carrying active Bluetooth phones in 2006, a number that has since increased sharply. “Recently, we’ve seen an astronomically higher number of Bluetooth devices out there,” he says.

While all phones allow you to turn off the Bluetooth functions, other technologies could also be leaking your personal data. You can follow individual mobile-phone handsets for as little as £5 per month (www.traceamobile.co.uk), or sign up to a Facebook application (www.sniffu.com) that can pinpoint your friends’ phones on a digital map. All of these require the permission of the phone’s owner, but a determined tracker can get round the safeguards – forgive us for not telling you how.

Another technology that allows identification and tracking is RFID. This short-range system is used to track goods in warehouses, to “chip” pet dogs and to secure the electronic passports that the UK has been issuing since 2006.

However, German researchers recently demonstrated that they could remotely scan the latest e-passports – through a coat or bag, for instance – and determine at least the holder’s nationality.

The risks of carrying these telltale gadgets shouldn’t be exaggerated. Just because someone can follow phones through a city, or pick UK citizens out of a crowd, doesn’t necessarily mean their owners are more likely to suffer a crime. In fact, says O’Neill, it might even be a benefit. “When people are offered good enough location-based services they’re going to embrace the technology even if that means giving up small pieces of personal data. Imagine a cafe that offers a discount to passers-by via Bluetooth, or a blue plaque that gives multimedia tourist information.”

Or then again, imagine being deluged by nonstop Bluetooth spam as you walk around – a bit like the billboards in the science-fiction film Minority Report, that scan your iris as you pass and address you by name.

Ominously, the Information Commissioner’s Office has confirmed that unsolicited Bluetooth messages are not covered by current antispam regulations. It may only be nipping at our heels at the moment, but be warned, Bluetooth can bite.

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