You've arrived at the physicians office, for reasons up to you, and knock on the vaguely ajar door waiting for someone to let you in as to not intrude. Merlin is the only one left, as Gaius the court physician has left for the evening to act on physician duties. Whether that was apparent to you or not, is up to yourself.

He hears knocking and hurries to put away the magic book while moving to the door "Yes, coming!" He opens the door and gives you a silent greeting