That point being made, I'll make an educated best guess as to what is going on with this warrant. The Presidential Records Act bars the outgoing POTUS from retaining any documents he has pertaining to his job as POTUS. Purely personal documents the POTUS may keep. The distinction of classified/unclassified is irrelevant in the PRA. If we're talking about taking classified documents, that would be something under the Espionage Act? So we could be looking at more than just the PRA, but it's hard to say.
The most important issue here is, if they thought this was possibly inadvertant, they would have approached this informally by contacting his lawyers and politely requesting the return of the records. So they would need to be pretty sure of Trump's intent or they would not handle it like this. If the records in question were a small percantage of a larger number of documents to which he was legally entitled, it could be hard to prove. Unless someone in Trump's orbit squawked, maybe told the FBI that he or she had been directed by Trump to retain certain presidential and/or classified records, and possibly even that the person in question warned Trump it was illegal but Trump told him/her to do it anyway.
Anyway, yeah, I think someone in Trump's orbit has spoken to the FBI. That's just a best guess of course.
