He guessed what had happened.

Somebody had seen the door ajar, and guessing from the fact that she had left it open that she was returning immediately, had slipped a piece of wood, which looked to be and was in fact the stalk of a match, between the catch of the spring lock and its sheath.
"What has happened ?" asked the girl in a troubled voice.
"Nothing," said Tarling airily.

"It was probably your disreputable step-father did this.

He may have lost his key." "He could have gone in the front door," said the girl uneasily.
"Well, I'll go first," said Tarling with a cheerfulness which he was far from feeling.
He went upstairs, his lamp in one hand, an automatic pistol in the other.
The stairs ended in a balustraded landing from which two doors opened.
"That is mother's room," said the girl, pointing to the nearest.
A sense of impending trouble made her shiver.

Tarling put his arms about her encouragingly.