Tracked by a Tattoo by Fergus Hume

Here, indeed, was food for reflection. That the instrument with which the crime had been committed should come into the detective's possession was extraordinary; but that it should have been left anonymously at the rooms of the murdered man was inconceivably audacious. Fanks at once returned to the chambers, and closely questioned Maxwell and Robert. It struck him that the latter might have had a hand in placing the mysterious parcel in the letter box.

"I examined the box an hour ago, sir," said Maxwell, "as you told me to look after all letters. There was nothing in it then. It must have been placed in it since."