The University of Uppsala (in Sweden?) has developed a black spherical robot that can roll around and patrol a location. Using radar and infra-red, when it detects an intruder, it sounds an alarm, summons helps, and follows the intruder, taking pictures. It's capable of travelling 20mph, faster than most humans, and is able to give chase over snow, mud and water. It's about 2ft in diameter, and weighs 10lbs, so there's no danger of accidentally harming (by running over, for example) false positives.

All they need to do now is add a red glowing eye/camera lens.