How does Sherlock Holmes know that Sir Henry was followed in London?

First, Holmes was told that no one knew of Sir Henry's visit, then.... Holmes followed Sir Henry and observed the man in the hansom cab following Sir Henry and Mortimer.

He quickened his pace until we had decreased the distance which divided us by about half. Then, still keeping a hundred yards behind, we followed into Oxford Street and so down Regent Street. Once our friends stopped and stared into a shop window, upon which Holmes did the same. An instant afterwards he gave a little cry of satisfaction, and, following the direction of his eager eyes, I saw that a hansom cab with a man inside which had halted on the other side of the street was now proceeding slowly onward again.