If he hadn't been deaf, he wouldn't have said that Miss Rider was the cause of his being wakened up.

I suppose it was something that happened outside." "What did he hear ?" asked Tarling quickly, and the porter laughed.
"Well, sir, he thought he heard a shot, and a scream like a woman's.

It woke him up.

I should have thought he had dreamt it, but another tenant, who also lives in the basement, heard the same sound, and the rum thing was they both thought it was in Miss Rider's flat." "What time was this ?" "They say about midnight, sir," said the porter; "but, of course, it couldn't have happened, because Miss Rider had not been in, and the flat was empty." Here was a disconcerting piece of news for Tarling to carry with him on his railway journey to Hertford.

He was determined to see the girl and put her on her guard, and though he realised that it was not exactly his duty to put a suspected criminal upon her guard, and that his conduct was, to say the least of it, irregular, such did not trouble him very much.
He had taken his ticket and was making his way to the platform when he espied a familiar figure hurrying as from a train which had just come in, and apparently the man saw Tarling even before Tarling had recognised him, for he turned abruptly aside and would have disappeared into the press of people had not the detective overtaken him.
"Hullo, Mr.Milburgh!" he said.