Mishandling is criminal. That's the rub here, bub. It isn't necessary for them to "know" they had mishandled it, only to be reckless with it, it is their responsibility to secure it, and mishandling it is criminal. If I mishandled these in the same way I would be in prison. Now, with Pence, like with Biden, stuff when when they were VP can find a way to your house during a briefing. They also have plenary power (since 2009) to declassify if they wanted. So, it is possible that those documents were a wash and were not still classified, though neither of them say they were. It is also possible that during their time as VP folks could bring it there and leave it because they broke the law and mishandled it.
However, if they were from their time in the Senate, that is no longer the case. There is no reasonable way the documents get there other than mishandling of that information by the person who was in the SCIF and cleared for it. Folks are not dropping by shedding classified information from the Senate SCIF.