This question already has an answer here:
In the Last Jedi, the Resistance, correctly assumes that
The First Order has a way of tracking them through hyperspace.
But why did they came to that conclusion, knowing that there is a technology that can signal your position across the entire galaxy?
Actually, Leia uses it so that Rey can find them
So why go straight to "a new technology has been developed" when the much simpler possibility "There is a tracker on one of our ships" (possibly hidden by a mole) was a valid possibility?