"I didn't know I was going to disappear." He had no intention of coming here. In fact, he hadn't even been using magic at the time, so he was certain it wasn't even his own fault he'd vanished. Something else had brought them here. He supposed it wasn't all bad, since at least all this could happen here, where Arthur really had no more power then anyone else. Even less power, actually, considering the number of sorcerers outnumbered him, and all of them seemed to look up to Merlin for some reason. What better place to have the prince get used to the idea of his being magic?
"And I would have told you...Eventually. I had other things to worry about though without adding on the fear that you'd suspect me of using the magic against you or simply telling Uther..." Arthur only had to deal with a small part of the problem most of the time. He didn't have to go after Nimueh, or deal with the manipulative lizard under the castle, or worry that saving an innocent child one day could lead to the prince's death in the future...