RFID tags are used to track all kinds of things and according to this Wikipedia essay, even people,

Radio-frequency identification (RFID) is the use of a wireless non-contact radio system to transfer data from a tag attached to an object, for the purposes of automatic identification and tracking. Some tags require no battery and are powered by the radio waves used to read them. Others use a local power source. The tag contains electronically stored information which can be read from up to several metres (yards) away. Unlike a bar code, the tag does not need to be within line of sight of the reader and may be embedded in the tracked object.

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Since RFID tags can be attached to clothing, possessions, or even implanted within people, the possibility of reading personally-linked information without consent has raised privacy concerns.

Now, French researchers have developed a means of making RFID tags even easier to attach and could further increase privacy concerns. From the Feb. 6, 2012 news item on Nanowerk,

Radio Frequency Identification (RFID) tags are an essential component of modern shopping, logistics, warehouse, and stock control for toll roads, casino chips and much more. They provide a simple way to track the item to which the tag is attached. Now, researchers in France have developed a way to deposit a thin aluminum RFID tag on to paper that not only reduces the amount of metal needed for the tag, and so the cost, but could open up RFID tagging to many more systems, even allowing a single printed sheet or flyer to be tagged …

The researchers used this technique,

There are several techniques used to deposit an antenna on PET: etching, electroplating; and on paper: screen printing, flexography and offset lithography. Now, Camille Ramade and colleagues at the University of Montpellier have demonstrated how a simple thermal evaporation process can deposit an aluminum coil antenna on to paper for use as an RFID tag. Aluminum is a lot less expensive than copper or silver, which are used in some types of RFID tag. The researchers suggest that the approach would reduce the cost of RFID tagging to a fifth of current prices, which could represent significant savings for inventory users operating millions of RFID tags in their systems.

For anyone who’s curious about PET, it’s a plastic substrate. From the NAPCOR website page about PET,

PET stands for polyethylene terephthalate, a plastic resin and a form of polyester.