Tracking devices have to have a receiver and integer solving processor to develop position. Then, they have to also have a transmitter, which will require considerable power to remain in operation.
Such a device could not be hidden under your skin.
A passive Faraday loop chip that could signal its presence to various sensors as one passes by them would be the closest thing to tracking that could be embedded in your skin.
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not with current technology but with alien tecnology they recovered in the spacraft in Roswell... ...just pullin your leg....LOL