You create an immobile, magical sensor that continually scans the area around it, looking for your enemies. The sensor has low-light vision, darkvision to a range of 30 feet, and the scent ability to a range of 30 feet. The sensor can make Perception checks (with a bonus equal to your caster level plus caster ability modifier) to note hidden or invisible creatures. If the sensor sees or smells any creature that presents a potential threat (from intelligent creatures sneaking up on you, to animals which might potentially attack if they stumble upon you by accident), it alerts you mentally provided you are within 120 feet, and is then discharged.