Every year, Reddit publishes a transparency report detailing how many requests for information it received, along with more mundane things like copyright takedowns. What Reddit can't say, by law, is if it's been sent any US National Security Letters, AKA secret subpoenas. Nod nod wink wink. Reddit can't outright say if it has received any NSLs, but it can not say that it hasn't received any NSLs. Confused? It's simple: in the 2014 transparency report, Reddit included the following paragraph:
In the 2015 transparency report, no such paragraph existed. The statement is known as a "warrant canary" — its absence implies rather heavily that Reddit has received a NSL since last year. Luckily, the distinction hasn't escaped Reddit's crack team of amateur sleuths, as evidenced by the top comment on the transparency report.
Stay suspicious, Redditors.