Why does Holmes replace the packet in the moat after finding it?

Holmes replaced the packet in order to see who would retrieve it after the moat was emptied. The person who had placed it there to begin with would be the obvious person to remove it.

This we accomplished by the very obvious device of announcing that the moat would be dried to-morrow, 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. We have no fewer than four witnesses as to who it was who took advantage of the opportunity, and so, Mr Barker, I think the word lies now with you.'