Deep in the Otay Mountain Wilderness, there is no wall... But it is fertile for an invisible kind of fence, one built of artificial intelligence, radar, drones, sensors, motion-activated cameras and even lidar, the technology used in self-driving vehicles... An electronic fence is not about preventing intrusion as much as it is about detecting intrusions and then intercepting them. While even the strongest proponents for such a technological solution admit that physical barriers are probably best in urban areas like San Diego and El Paso, Texas, they see a virtual wall as a cheaper and more effective way to police...