"I see, Watson.

You are sketching out a theory by which everything they say from the beginning is false.
According to your idea, there was never any hidden menace, or secret society, or Valley of Fear, or Boss MacSomebody, or anything else.
Well, that is a good sweeping generalization.

Let us see what that brings us to.

They invent this theory to account for the crime.

They then play up to the idea by leaving this bicycle in the park as proof of the existence of some outsider.