> There are far more obvious ways to know if someone is home, from thermal sensors, looking in windows and knocking on doors, or park outside and just watch.
All of these have the disadvantage that the observer can be spotted - there exist numerous articles on how to spot cameras because sketchy AirBnB hosts placed hidden cameras in bedrooms and toilets. A radio-based monitor however doesn't need line of sight - if small enough, it can be hidden just about anywhere where no one looks, like the interior of a rainwater tank, under the bottom of a trash can, in a roof water drain.
All of these have the disadvantage that the observer can be spotted - there exist numerous articles on how to spot cameras because sketchy AirBnB hosts placed hidden cameras in bedrooms and toilets. A radio-based monitor however doesn't need line of sight - if small enough, it can be hidden just about anywhere where no one looks, like the interior of a rainwater tank, under the bottom of a trash can, in a roof water drain.