I know this is a facetious question.. but...
ASSUMING that you could get a regular supplier-- AND-- that you have completely overcome all of the technical obstacles to using
the technology (such as being rendered inert through the accumulation of biological polymers, like protein complexes such as blood platelets) -- One could produce a device capable of sending a very simplistic cellular signal, that relies on GPS signals. Extreme low power computing is totally a thing, and as such, theoretically possible...
However, you then end up with certain physics problems manifesting themselves. Namely, you cannot produce reliable signals without an appropriate antenna geometry, and you do not get appropriate antenna geometries without certain size requirements being met. (at least, not for anything outside of the near field.)
SO-- If you don't mind your implant recipient looking a little like the hunchback of notre dame, due to the enormous bulge on his shoulder from the excessively large implant, that is excessively large because of all the requisite physics needed for the devices to operate--- Then sure-- SURE-- you can put something like that inside somebody.
Considering that the previous implants are about the size of a pine nut, and would be easily felt beneath the skin of somebody that had one inserted-- (and would CERTAINLY feel it going in!!)-- I cannot imagine a circumstance where such an enormous implant could be installed surreptitiously.
Mostly, it's just people basing a world view on faulty modern-reinterpretation of really old bronze age prophetic rambling, who cannot be bothered to actually research anything at all about the potential obstacles to feasibility of such an absurd conjecture, but respond very fearfully of the notion instead.
As NG properly points out, the human body is a VERY hostile environment for ANYTHING that gets implanted. The body does NOT like foreign material, of any kind. Things like artificial joints, osteopedic supports, or artificial heart valves, *ALL* have to be made of very special materials to avoid having the body freak its shit, and destroy the surrounding tissue with runaway inflammatory reactions, or (in the case of heart valves) coating them in thick layers of biopolymers (which then can lead to stroke and other complications). This is why the pet-tracker implant is hermetically sealed inside a borosilicate capsule.
There *ARE*
vastly smaller RFID tags that have been made-- some as tiny as particles of black pepper-- with some suggestions that they could be integrated into paper currency notes to better track and prevent counterfeiting-- They are *NOT* made to the necessary requirements to survive inside a biological organism's body, and would be inactivated in hours by a host if they were just loaded up and shot under the skin. (In addition to likely causing all manner of adverse reactions.)
BUT-- these people are a lot less interested in the danger of being tracked-- (See cellphones, Facebook, Targeted advertising, Et Al) and MUCH MORE interested in the religious prognostication (Mark of the beast, and eternal damnation)-- so who am I to try to bring enlightenment, where none is wanted?