That technically isn’t true. All government material is subject to classification, even that rating is ‘Unclassified’. However, unclassified does not mean you can ignore the rules and regulations for storage and dissemination of the material: unclassified just means that the information has been assessed as having minimal impact to the reputation, security and operation of the home (or a foreign) government and the parent department from whence the information sprung if it was released. There is still the doctrine of ‘need to know’. You know the saying “Just because you can, doesn’t mean you should”? Information security works the same way: just because Person A is rated to top level, that doesn’t automatically give them the right to view all material at that rating or even below it in fact. Generally they will be able to view material directly relevant to their job, with anything broader requiring requests, justifications and approvals.
No government information, no matter how innocuous, had any business being stored on a mail server in an airing cupboard, or mixed in with personal documents in a basement. Both of these things are grossly negligent handling of information and should be, or should have been, prosecuted very vigorously.