Assume it's really true that the NSA has no idea what documents Snowden took, and that they wouldn't even know he'd taken anything if he hadn't gone public. The fact that abuses of their systems by NSA officers were largely discovered through self-reporting substantiates that belief.
Given that, why should anyone believe that Snowden is the first person to walk out the NSA's door with multiple gigabytes of classified documents? He might be the first to release documents to the public, but it's a reasonable assumption that the previous leakers were working for Russia, or China, or elsewhere.
Via Ben Brooks