Apple is one of several companies whose transparency reports contain these warrant canaries — Apple's dates from November 2013. They became more widely used after the revelations of mass surveillance brought to light by Edward Snowden.
The premise of a warrant canary is that Section 215 of the Patriot Act can compel companies not to tell anyone about being served with a warrant, but that the law can't compel a company to lie and say that it hasn't received a warrant. This has not been tested in court yet.
It seems likely, based on the latest report, that Apple has now received at least one of the secret surveillance requests.
The warrant canary's disappearance is significant because Section 215 of the Patriot Act permits the National Security Agency to demand companies to hand over their business records in secret, and is believed to be the legal foundation of the controversial PRISM program, which forced major tech companies like Google and Yahoo to participate in a data-collection scheme.
The Patriot Act tool is also controversial because the NSA gains permission to use it by applying to the FISA Court, a body where only the government can speak and whose records are kept almost entirely secret. The tech industry has been battling to disclose the existence of so-called "FISA requests" and only won the right to do so this year; however, companies must wait six months to disclose the number of requests they receive, and can only do so as a range (such as "0-999″).
Apple's "warrant canary" disappears, suggesting new Patriot Act demands [Jeff John Roberts/Gigaom]
(Image: Jean-Baptiste Greuze – A Girl with a Dead Canary, Wikimedia Commons/Public domain)