Why did Holmes want to look at the hotel register, and what did he find?

Holmes visits the Northumberland Hotel, where Sir Henry Baskerville is staying. Examining the register, Holmes pretends to know two of the hotel's visitors, and fools the clerk into revealing information about them.

"Sir Henry Baskerville is upstairs expecting you," said the clerk. "He asked me to show you up at once when you came."

"Have you any objection to my looking at your register?" said Holmes.

"Not in the least."

The book showed that two names had been added after that of Baskerville. One was Theophilus Johnson and family, of Newcastle; the other Mrs. Oldmore and maid, of High Lodge, Alton.

"Surely that must be the same Johnson whom I used to know," said Holmes to the porter. "A lawyer, is he not, gray-headed, and walks with a limp?"

"No, sir, this is Mr. Johnson, the coal-owner, a very active gentleman, not older than yourself."

"Surely you are mistaken about his trade?"

"No, sir! he has used this hotel for many years, and he is very well known to us."

"Ah, that settles it. Mrs. Oldmore, too; I seem to remember the name. Excuse my curiosity, but often in calling upon one friend one finds another."

"She is an invalid lady, sir. Her husband was once mayor of Gloucester. She always comes to us when she is in town."

"Thank you; I am afraid I cannot claim her acquaintance. We have established a most important fact by these questions, Watson," he continued in a low voice as we went upstairs together. "We know now that the people who are so interested in our friend have not settled down in his own hotel. That means that while they are, as we have seen, very anxious to watch him, they are equally anxious that he should not see them. Now, this is a most suggestive fact."