In the US, certain types of warrants can come with gag orders preventing the recipient from disclosing the existence of warrant to anyone else. A warrant canary is basically a legal hack of that prohibition. Instead of saying “I just received a warrant with a gag order,” the potential recipient keeps repeating “I have not received any warrants.” If the recipient stops saying that, the rest of us are supposed to assume that he has been served one.
Lots of organizations maintain them. Personally, I have never believed this trick would work. It relies on the fact that a prohibition against speaking doesn’t prevent someone from not speaking. But courts generally aren’t impressed by this sort of thing, and I can easily imagine a secret warrant that includes a prohibition against triggering the warrant canary. And for all I know, there are right now secret legal proceedings on this very issue.
Australia has sidestepped all of this by outlawing warrant canaries entirely:
Section 182A of the new law says that a person commits an offense if he or she discloses or uses information about “the existence or non-existence of such a [journalist information] warrant.” The penalty upon conviction is two years imprisonment.
Expect that sort of wording in future US surveillance bills, too.