Raphael.

After witnessing the nth interrogation of Sebastian with the Sword, he could not stand it anymore. Was it possible that Shadowhunters needed to hear those same things over and over till oblivion? Magnus got convinced it had to be just a form of torture for the boy, than an actual useful act.
He had no intention to spend whole weeks there in Alicante, knowing the poor Raphael could be anywhere in the world, lost and scared and with no memory of any of them nor of the Invisibile World. He could guess that depriving him of his immortality had yo probably turn him back into a mundane, which was positive, to an extent.
He was no longer a vampire, but he had been, people knew who he was and he could receive dangerous visits from people he would be unable to defend himself from.
It was of critical urgence to find him, at that point, he was nothing more than a helpless 15 year old with some serious confusion.

He sneaked out of the Council Hall wiyh Alec and they started to walk towards the Hill just outside of Alicante, where the Warlock would be able to create a portal to use to return to New York. They needed some of Raphael's belongings in order to track him with magic and they would only find some at the Dumort Hotel.