In Duncan's perception, two anomalies had caught his attention.
One of the markings, of course, came from the Oak.
The ship, which he had once engulfed in flames entirely, was still emitting a strong "presence," like a blazing fireball wandering within his senses, yet he still couldn't pinpoint the ship's actual location—every time he tried to determine the Oak's position, he would get a strange and blurry result. This result indicated that the Oak was somewhere next to Frost City-State, a place already blockaded by the Mist Fleet and Frost Navy.
The other "problematic" marking was on Gatekeeper Agatha.