Hux and Snoke allude to “having the resistance tied to a string.” Were the First Order locked-on to Finn-2-187 and tracking him through hyperspace? Since Finn had been abducted as a child and in their ranks as a stormtrooper for some time before The Force Awakens, did the First Order embed trackers into their soldiers and more easily seek out defectors?
They would have noticed when he left the ship and traveled to Canto Bight