Who had put the bundle in the moat?

Mr. Douglass put the bundle in the moat. From the text, we can infer that the urgency to withdraw it can be found in the fact the moat was supposed to be dried the next day.

“It was of the first importance, however, that we should be able to prove who placed it there. This we accomplished by the very obvious device of announcing that the moat would be dried tomorrow, which had, of course, the effect that whoever had hidden the bundle would most certainly withdraw it the moment that darkness enabled him to do so."

Cecil Barker had put it in the moat and Holmes wanted him to take it out himself while draining the moat. Barker was caught red-handed when he took it out.