The Dark Eyes of London by Edgar Wallace

That sense of security, of peace, and of deep happiness was inexpressibly sweet, she thought, as she lay in Larry Holt's arms. Presently she released herself, and in a few hurried words had said enough to send a small battalion of detectives racing to the dormitory and through the brick door, which she had left ajar.

Larry handed the girl to the care of Harvey and followed his men. The room where Diana had been imprisoned was empty. He stopped long enough to switch off the current, then joined the searchers. There was no doubt that this place, for all its unoccupied appearance, had regular tenants. They found rooms that had been built within rooms, a thin wall being erected within a few feet of each window. This meant that the house might be occupied at night, and lights might blaze in every apartment without anybody outside being the wiser.