Since knowing Merlin, Arthur had... actually made it more of a point to actually knock before entering (unless he couldn't be bothered, in which case he didn't; his need to prove himself only extended so far. ... well, on this one, anyway) and so his presence was announced by a polite, firm knocking, numbering three (not four, not two, and five was right out) and a polite not-bursting-into-the-door.