Since Leia has a beacon or tracking device that allowed Rey to track her in Star Wars: The Last Jedi, why didn't the First Order use that technology to track the Rebel fleet? Leia's bracelet must have been emitting some signal for Rey to trace, so the First Order must have had some way to detect Leia's device's frequency, and could have tracked the rebel fleet by tracking Leia's bracelet. But they apparently did not. Why not?
One clue comes from the movie, when Leia first shows Finn the beacon. He immediately recognizes it and says it's a "cloaked binary beacon".
The Last Jedi Visual Dictionary also says this about the beacon:
Leia wears a compact S-thread transmitter bracelet, with Rey holding the mated pair. In this way, Rey will be able to find the ever-mobile Resistance fleet once she recovers Luke Skywalker.
So the assumption is that the signal is (A) cloaked, or encrypted so that others can't read it, and (B) permanently linked to 2 physical devices, which are the only ones that can read the signal.
I was under the impression that it was some sort of quantum entanglement-type technology which couldn't be tracked except by the other part. I have no canon reference for that, but I feel like it was implied in the following ways:
- It couldn't be hacked or altered - If it could Finn would have changed the location so Rey didn't warp in to the Snoke attack.
- It couldn't be deactivated - Or Finn would have deactivated it.
They didn't know Leia had a beacon. It's possible that if they had known about it, they could have detected the frequency and tracked it (or that they could have checked all frequencies and somehow found it). Since that idea is not addressed in the movie, it's also possible that it's somehow magically untrackable unless you have the other one that Rey had.
They also didn't think such a thing would help them since they were able to track the big ship anyway, so they most likely wouldn't have used it even if they had known about it.