trying to find or make a very light/small chip (rfid, bluetooth, etc) that can send its location to a cell phone if lost
I don't know if it will be battery powered and what duration of battery do you need but here are a fe options I have used:


NRF24L01 You can modify transmission power and transfer speed to use less power. It is an RF 2.4GHz transceiver, if you wan't to connect to an smartphone It won't be possible.
Bluetooth SMD Module - RN-41 It is a very small Bluetooth module so you can connect to an smartphone or PC, it uses about 40mA depending of bit-rate and distance.
Bluetooth SMD Module - RN-42 It is the same as RN-41 module above but it has a reduced transmission range (50-60 feets) so it uses less power too and also is cheaper.


If you can tell me more about how you will power the system and the duration of the battery if you will be using one I can think more options :)

About location, please give more information, what precision do you need ? What do you mean by small ? 50 x 50 mm is ok or you need something smaller ?