...In fact that is a problem with every hidden tracker... for GPS to work, and for it to be able to transmit data, it should have an external antenna. And that means that the device can be found by a crook and be removed...
Well, not exactly as I mentioned...
GPS is simply a one way system that broadcasts a static signal from geosynchronous orbiting satellites, that devices receive; devices generally need four signals from four different satellites. The receivers "do the math" with real time clock data, and "triangulate" the global position to within a few meters. (If you are the US Military they'll get it the accuracy within inches). The receivers and logic are cheap, they are built into almost every phone, and even a lot of very inexpensive cameras. Remember cameras? They were these things that took pictures before smart phones.
For the trackers to work, they need an uplink system, such as your aforementioned SigFox system (never heard of that, but it
is apparently in the USA) but more likely they all use a wireless provider network. You don't need large visible antennas; the antennas in all smart phones today (as opposed to the old analog phones with actual antennas) are hidden inside. Ditto for those in the cars. It is because they all use the cell phone network that they require a monthly fee.
These location-finding devices sometimes use a combination of GPS and triangulation from the cell phone towers.
Most of the better equipped Ford products today have built in geolocation, and I would defy one to find the hidden antennas. Shouldn't matter much, there are so many Audis, Mercedes-Benz, Range Rovers, and far more desirable for thieves cars around I certainly don't need to worry much about our Fords. Mostly they'd want the wheels off a Mustang, the airbags or maybe the CAT. You don't need to steal the car for any of that. But, if one were to be stolen I could tell the police in a heartbeat precisely where it was.
The pioneer of the car tracker was LoJack.
https://www.lojack.com That's the only system directly tied to law enforcement. I believe they use a proprietary system on radio bands dedicated to law enforcement, not on the cell phone bands. The inventor was a Kojack lookalike who used to live in the town I lived at in Massachusetts.