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Knocker

Decency impelled him to retrieve the key and go up and look. The room appeared empty, but he could trace the knocking now to its source, which appeared to be somewhere near the ground between the bed and the dresser. Feeling about on the wall led to the discovery of a dent, and judicious pressing in its center led to the sliding back of a good six feet of wall to reveal a space from which rolled out a woman in a sweater and dirty slacks.