Trump reluctantly returned the first 15 boxes to National Archives, he wouldn't have done that if it wasn't theirs.
As for the recent 12 boxes, we don't know what was in them, but we do know that the National Archives, DOJ, FBI, and the Judge that signed off on the Warrant, believed Trump was holding out and didn't return all the documents previously. (Pretty easy for the National Archives to determine there are missing documents, gaps in the timeline, etc.)
The return of the first 15 boxes showed Trump took documents he wasn't legally entitled to take.
Depending on what they find in the recent 12 boxes may be the catalyst for further action against Trump.
Noting, Trump shouldn't have taken any documents he wasn't legally entitled to take in the first place.
"...A President has 5 years to go through documents while archives sorts out who retains copies...."
Don't confuse the Ex-President being able to request specific document not be archived, with the Ex-President taking documents he wasn't legally entitled to take.
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Presidential Records Act (PRA) of 1978
Establishes that Presidential records automatically transfer into the legal custody of the Archivist as soon as the President leaves office.
