A solitary knock, and Mr. Darcy steps in alone—why did he come when no carriage was heard?

Elizabeth was sitting by herself the next morning, and writing to Jane, while Mrs. Collins and Maria were gone on business into the village, when she was startled by a ring at the door, the certain signal of a visitor. As she had heard no carriage, she thought it not unlikely to be Lady Catherine; and under that apprehension was putting away her half-finished letter, that she might escape all impertinent questions, when the door opened, and to her very great surprise Mr. Darcy, and Mr. Darcy only, entered the room.