A number of cities now have a technology called shotspotter. It's basically a network of acoustic sensors on rooftops that are designed to detect gunfire and alert the police. Many big city police departments have a major problem where a shooting will take place and the cops won't find out about it until someone stumbles into a hospital, at which point catching the bad guys becomes much harder. Lots of people are unwilling to call 911 either because they fear retaliation for snitching or they don't trust the cops or they're apathetic and they don't think any good will come from reporting a crime. I imagine one motivation for this twitter scanning technology is similar - if they're monitoring tweets in real time they can get a precise location of a shooting and also the twitter handle of a witness.