That itself is a specious argument. You know about the documents that Trump returned...because he returned them. The FBI, executing its search warrant, found 102 documents. Speculation on what the boxes could have contained is just that. Speculation. And treating speculation as fact is inuendo.
To get a search warrant, you need more than speculation, you need probable cause. The FBI was able to convince a judge they had probable cause to believe that Trump still had more classified documents that he hadn’t turned over. They based that in part on video tapes showing boxes being removed before they could be searched by Trump’s attorney, and by the fact that the boxes that were not removed did have classified documents. Since the search warrant did recover another 102 documents, it seems that they were correct.