...RFID tags of the kinds which could be used for implantation in humans (or clothes and other items) are passive.
They will give off the information stored on them (usually a single number) only when activated.
The tag has no powersource, the power to transmit is derived from the signal which requests the information.
When the signal is received the chip gets power. If the signal matches a signal the tag is programmed to respond to it transmits the information stored in it until the signal dies down.
Typically the range of RFID scanners is limited to a few meters at most. Higher powered tags require independent power sources (batteries), making them impractical for longterm use (how to replace the battery regularly?), larger (and thus more cumbersome to install and carry around), and far more expensive (given the now estimated 300 pound pricetag for the UK biometrics ID card they're trying to force on every resident that's clearly not a problem for governments, they just pass on the bill to the taxpayer/victim).
The myth that RFID tags are no larger than a grain of sand and cheap enough to install in penny items like pencils and candy wrappers is also just that, a myth.
At current the tags are still about a square centimeter in size and maybe a tenth of a millimeter thick for the smallest of them...
The PENI tag is a passive tag with an embedded antenna. It is only 2.2[mm] on a side and claims a range of perhaps as far as 100 meters.
http://www.discover.pitt.edu/media/p...1_PENITag.html
RFID DETECTOR NETWORKING
Current RFID detector systems like toll booths, inventor control, and delivery services could be networked to form such an integrated network. In the future RFID cash registers and cell phones could be integrated. PENI tag compatible systems would likely have greater range.
http://www.nokia.com/nokia/0,,55739,00.html
The information such a RFID network would gather is very valuable. It is unlikely that such a potentially profitable system will not be implemented.
RFID AMBER ALERT
All of the networked detectors could be instructed to alert the police if they detect the unique RFID code for the victim’s DNA samplers. Such a system could provide a fast acting and wide spread active search for the victim’s DNA samplers.
DNA DATABASES
Kidnappers, rapists, and assailants would be venerable to this network through the RFID tag in the DNA samplers. The recovered DNA would be fed into a DNA database for matching or stored as a cold case for future matching. It might be possible to identify a suspect with a ‘near hit’ on a close relative already in the DNA database.
Whether the RFID network would be used abusively or not is not affected by using it to locate DNA samplers. The RFID network might as well be used to identify violent criminals. Otherwise, it is just a wasted opportunity to prevent violent crime. Kidnappers, rapists, and assailants will go free and their victims will not be rescued.