Warrant Canary

Canary in a coal mine

In early 2015, Reddit published a transparency report that contained heading for National Security Requests, noting, "As of January 29, 2015, reddit has never received a National Security Letter, an order under the Foreign Intelligence Surveillance Act, or any other classified request for user information."
Five hours ago, Reddit published its 2015 edition, which contains no mention of classified requests for user information.
"Warrant canaries" are a response to the practice by governments of serving warrants on service providers that include gag orders forbidding the service from disclosing the warrant's existence. By publishing regular transparency reports listing the number of secret warrants received to date as "0," then omitting the section dealing with those warrants once the first warrant has been served, companies can create a kind of dead man's switch -- done right, it might even stop governments from even trying to serve a warrant (except in Australia, where warrant canaries are illegal).
Reddit had a national security warrant canary in 2015. It's gone now [Chris Soghoian/Twitter] 

It's all over the net. I found this explanation on BoingBoing. Via Marcel on Monday Evening and Bruce Schneier on Security