Merlin didn't know that Zach had followed him until he felt a hand tug his shirt; he jumped and fought to pull away, his heart racing. "Let go of me!" he said, insistent and sharp with something that wasn't all anger.
He'd had enough of being followed back in the Muggle world - being approached like that from behind didn't make him feel particularly comfortable, and then there was the fact that he thought Zach was going to deck him.