Biometrics my dear boy.

As hotty says they're watching you and they [GOV.CO.UK] know where you are and where you've been. It's similar to tags you get on music CD's. The information they carry can be read and located to build up a profile of what you do, where you live, and how you spend your money. Best advice (excluding your passport) is rip the tag off the product before you pay for the item (offer to buy as damaged goods at retail price). However, outlets such as Argos or Homebase Group, Vigin, HMV, and numerous clothing retailers will try and stop you doing this before you pay or leave the shop.

P.S These tags can also be stiched into clothing labels.