Subject: Re: The specail counsel's report
Any guesses as to how these documents got out of a SCIF undetected while old Joe was a Senator or Vice President?

The most likely explanation is that some staffer took them and included them in boxes of other, unclassified documents. It's exceptionally unlikely that a U.S. Senator or the Vice President would have packed up his own stuff on leaving office - it certainly would have been delegated to a subordinate staffer.