my apologies for assuming you knew what I meant by "secret" and "top secret". When information (be it technology, strategy, etc) is deemed to be of a sensitive nature, it is considered classified. And there are levels of classification; Proprietary (usually this has nothing to do with national security, but it's sensitive information for a business. If you knew Apple Computer was going to by company Foo before the public announcement, that would be proprietary information that a competitor would love to know), "Secret" and "Top Secret" (these levels pertaining to national security).
So while a bank's vault is secured, what it contains is not sensitive information (it's money, it's bearer bonds, gold perhaps...). You can know what is in there and it doesn't compromise the nation.
Where-as if you know some specific technical detail about uranium enrichment, you could sell that to another foreign nation, and THAT would compromise national security.
I hope this makes some sense!