They were scattered around his office in the open AFTER the FBI raided his home, Coyote! Before that they were secured in a locked storage room with two locks on the door per the FBI.Trump had no right to any of those documents once he left office. As for “secure location”, they were loose, scattered around his office in the open. Anyone could have seen them.
As for Trump's "rights"? That hasn't been determined in court. President's are the "ultimate authority" on classification of documents. Whether or not Trump had the authority to declassify what he had in his possession is a legal issue for the Supreme Court to decide.