A voice called Arthur during the night. He did not know what it was, but made him curious. If he was still the old weak Arthur, then surely he wouldn't go out alone in a dangerous place like that island.
The thing is, he's not weak anymore. Arthur is a mighty dragon, and there was no way common magical beasts would scare him. With that in mind, he stealthily left their improvised shelter and followed the mysterious voice.
"Don't worry about me. I'll be right back." thought Arthur. Kai and Sully were in deep sleep, tired of the 7 day trip, especially Kai, who was not a fan of ships, so they did not sense Arthur's departure.
The forest was so dense it almost had no lights. Even the moonlight couldn't touch it. Only crazy people would go out alone during the night, and people with night-vision, like Arthur.
He walked through the woods carefully to not draw too much attention, or the beasts that hunt during the night would get to him.