BusinessInsider reporting, here. Gee. How about fifteen thousand emails the Clinton lawyers did not turn over to State because, clearly in good faith, their caption search algorithms simply were inadequate for the task of winnowing wheat from chaf. No smoking gun lost to public attention that way - no smoking gun intentionally withheld; just a simple mistake, simply made, simply approved by the lawyers' client as the right way to do things.
UPDATE: WaPo, same topic, saying 14,900 non-classified previously unreleased emails the FBI recovered once it had the Clinton server in its custody.