The Safety Pin by J.S. Fletcher

Hackdale was essentially cool and calculating in temperament and disposition, and when the first thrill of his discovery had spent itself, which it did very quickly, his natural calmness in dealing with a difficult situation came to the surface. He had no doubt whatever that the body lying at his feet was that of the man who had so mysteriously disappeared from his room at the Chancellor Hotel. He had no doubt, either, that the man was dead. So much, he knew, was certain. And what was another, and to him a much more pertinent certainty, was that the man was lying dead in an unfrequented waste which lay within a hundred yards of Ashenhurst House--the private residence of Mrs. Champernowne.

There was just sufficient light--twilight--left to see what Hackdale saw, but he had the means of more accurate vision in one of his pockets.