Yeah, they tried to cover it up. They were hopeful they could get the documents back into safety quietly and without the notice of anyone that might hold them accountable.
I am telling you what really would have happened if I did this even by accident when I was working in the SCIF while in the Navy. I absolutely and unequivocally would have served time for it. Now, Presidents and VPs are a bit different, documents follow them around for briefings, material used during the briefings must be returned by the person who took responsibility for them to give the briefing. To me it appears as if he asked for them to leave the documents with him and took responsibility then simply lost track of where he placed them... However he had documents from before he was either the VP or the President... There is where he is going to find no excuse is good enough.
Senators and Congresscritters have access to the documents in the SCIF in the basement of the Capitol, when they have Need to Know (a legal term) and they must follow the rules of the SCIF. No electronics allowed, no images or copies can be made, no transcripts or notes can be taken from the SCIF,
When I worked on such material daily in the Navy I had a file, I could take notes, I had to mark the classification on the top and bottom of the cards I took notes on and at the end of the day my notes were held in my file in the safe in the SCIF.