Latest update:
So it appears this has been an ongoing exercise to retrieve these documents from Trump. National Archives were negotiating with Trump to return the documents and he finally returned some back in January.
He didn't return all though so he was subpoenaed around May to return more documents. In response to the subpoena, Trump returned another 15 boxes and his lawyer signed a letter confirming there were no more Top Secret documents left at Trump's house.
This is where things get a bit murky - Fox is claiming that the investigators knew there were more documents there because they asked Trump's aid to lock the room, but then why did his lawyer sign an affidavit saying there weren't? They also argue that the National Archives should have subpoenaed Trump again for the rest of the documents, but the documents belong to the National Archives so they shouldn't have to subpoena him at all.
This is a very interesting discussion: