We know of at least two: one reaches public court records. The other level we know about, for sure, is NSA level stuff, which remains secret forever as a matter of national security. Do others exist? For example it's possible an agency could use the information to gather internal investigation evidence, and keep that evidence dark only using it for further gathering of more traditional evidence. That way their techniques would remain secret.

Anyway, the technique of discovering government agency abilities through court records seems to be a vital part of our civilian rights that I don't see enough information about. If anyone knows anything about it and reads this, please share some info!