An alarm sensor can be placed near a doorway, or multiple can be placed around the perimeter of an area. Each covers an area 30’ across. When the area is breached (they can be set to ignore certain people or species, and to distinguish by size category) they set off an alarm. The alarm can be audible, or it can privately alert an individual. It’s a Difficult [16] check to detect an alarm sensor, and the same difficulty to sneak past it or disable it.