PhasmaFelis: I'd not be at all surprised to hear that an agency had ordered replacing the signs stating that the government had not ordered monitoring with the signs saying that the library could neither confirm nor deny. Even if monitoring had not been ordered, to deny that the monitoring had been ordered is a serious security breach. If an agency has the power to gag those it monitors, then surely it has the power to gag those it does not monitor, to avoid just the sort of scenario that you hint at.
And after all, where national security is concerned, freedom of speech is meaningless. National security is a matter of life and death. Dead people have no freedom.
And I can neither confirm nor deny whether any of this post is ironic.