Warrant canary - Definition from the Hotspot Shield Glossary
A Warrant Canary is a form of negative pronouncement that allows a communication service provider to alert its users to the fact that they have been served with a secret subpoena.
Secret government subpoenas are issued against those businesses who are suspected of being involved in terrorism, agents of a foreign country, or anyone who may have sensitive information relating to terrorist investigations and counter-intelligence.
A Warrant Canary will be posted on a website stating for example that “The FBI has not been here - watch closely for the removal of this sign”; that statement may be backed by a list of dates. As we can see, when this notice is removed, it will be clear that the business is indeed being investigated by the FBI. While that notices remains in place, and the dates stated are current, one can assume that a business has not received a secret subpoena.