According to Ars Technica, police have been using products like BlueJay that allow to stealthily "search all your tweets, then present results to cops."
[...] consider BlueJay, the "Law Enforcement Twitter Crime Scanner," which provides real-time, geo-fenced access to every single public tweet so that local police can keep tabs on #gunfire, #meth, and #protest (yes, those are real examples) in their communities. BlueJay is the product of BrightPlanet, whose tagline is "Deep Web Intelligence" and whose board is populated with people like Admiral John Poindexter of Total Information Awareness infamy.
BlueJay allows users to enter a set of Twitter accounts, keywords, and locations to scan for within 25-mile geofences (BlueJay users can create up to five such fences), then it returns all matching tweets in real-time. If the tweets come with GPS locations, they are plotted on a map. The product can also export databases of up to 100,000 matching tweets at a time.
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