From ProPublica:
A proposed rule to the Freedom of Information Act would allow federal agencies to tell people requesting certain law-enforcement or national security documents that records don’t exist – even when they do.
Under current FOIA practice, the government may withhold information and issue what’s known as a Glomar denial that says it can neither confirm nor deny the existence of records.
Also see National Press Club urges Administration to reconsider draft rule on Freedom of Information.