[The Arthur? It's possible Sebastian wouldn't know about him, but when you live with a little would-be hunter who knows his hunter history...]
Oh, I should hate to keep you! Please let me accompany you a bit of the way. I will not slow you down. [In fact, he's already falling into steps with Merlin without waiting for an answer. Hi, they're walking partners now (even if it's not that Arthur, errand boys are usually good for gossip).]