I'm looking for methods for preventing loss or for finding lost
property (including pets) and lost persons.

A weblink to the method / service will suffice.

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Clarification of Question by tateti-ga on 03 Jan 2006 09:34 PST

I mean:
A weblink to the methodS / serviceS will suffice. 
(I added the "S" because I expect more than one method / service)
Regards,
tateti

The implanting of microchips in pets can be helpful in reuniting the
animal with its owner:

http://www.pettravel.com/Pet_Microchip.cfm

That should work just as well on humans, except for the "1984" aspect.
 (Wonder if any intelligence services use it to keep track of their
agents.)

More seriously, in some places in the world, cellphone networks do
provide a facility to locate lost phones and users.