Without assuming superscience*, what would be the best way to duplicate the function of an
ankle monitor that will not be detected, even by a fairly thorough visual** search of a person?
The ideal solution, for the cool factor and because the GM will accept a locator as more plausible than an implanted bomb, but still wants the PCs 'tagged' by their scary, involuntary bosses, would be an implanted, subdermal radio beacon that is put into the back of their neck or somewhere equally hard for them to reach.
I'm not sure about the technological possibilities here. How realistic is real-time or near real-time exact satellite locating of a transmitter small enough to be implanted like an RFID microchip in animals?
As I suspect the answer will be 'not very', the
absolute minimum performance is being capable of giving submitter's location within 30 feet or so, updated with a burst transmission every minute that can be received within a reasonable range by someone with sensitive radio gear, like maybe within 10 miles (at least 2-3 miles are necessary even in urban terrain). This can rely on an aerial receiver, on board aircraft or UAV.
More precise location, more frequent updating and longer detection range would all be good to have, but not at the cost of having to use something too large to implant like a RFID chip.
48 hours of endurance without changing batteries or otherwise recharging are an absolute requirement, with a week being preferable, but normal operation would probably assume recharging by inductive charging every night, if only to keep it fully charged.
Does anyone have ideas about what kind of performance would be plausible and how best to go about this?
*Though, as this is being used by a powerful, super-secret, fictional joint task force under the US government, with need to know access to most or all technological research carried out by DARPA, HSARPA and probably a lot of other places, it's fine to assume cutting edge technology, not currently commercially available, as long as it currently possible.
**Obviously, electronic signals from it will be detectable and various scanners and sensor technology could always find it.