He walked around it, examining the vines until his eye caught a torn creeper and a white scratch on the paint.

It had been an outside job after all, and two weeks had already been lost.
Deduction was responsible for a mistake which would not have occurred had a little knowledge been acquired first.

That is the lesson of this story.
The denouement, which has no lesson at all, is interesting.

The superintendent saw no prospect of getting back the necklace, but before so informing the client, decided to cogitate on the matter for a day or two.

During that time he met by accident a friend who made a hobby of studying yeggmen and criminals and occasionally doing a bit of the amateur tramp act himself.
"By the way," said the friend, "do you ever hear of any 'touches' up the river or along the Sound ?" "Sometimes," answered the boss, pricking up his ears.