I have seen a video clip recently of a couple of taliban or whatever, firing a mortar. The clip says that we now have a tool that can track back to the mortar tube after “listening” to as few as three mortar rounds being fired from a single location. These towel heads are filming their actions, and sure enough, as they drop round #4 down the tube an incoming round kills both of them while the camera rolls. Kinda cool!!!
That's done using millimeter wave radar. The technology for back-calculating the launch site of a mortar round has been around since the '80s too. The Israelis use it all the time. It doesn't work against bullets, unfortunately, because they're too small to reflect even millimeter-wavelength RF energy. There have been some experiments on using infrared or visible light to track bullets back to their source, but to the best of my knowledge it's too delicate, sensitive to dust and dirt, and it doesn't have enough resolution to spot a bullet far away that's not aimed almost right at the sensor.
Here's how you'll know if they really have this technology perfected: when you start seeing high-resolution video of meteors burning up in the atmosphere, and this video shows up on YouTube. An optical sensor that could resolve individual bullets and track them could also resolve and track an incoming meteor at fairly high resolution.