Quite an awesome usecase would be to use this for perimeter security, like securing your house and cars. In my locality camera's are in a legal grey zone: the police applauds them for use in case of crime, but because of privacy laws you're not supposed to film other peoples houses and whereabouts. This would enable you to track movements without camera's. Software would make it easy to only give you signals when somebody or something moves on your property at night. All I would need is some machine learning to learn about rabbits, cats and foxes. Or you could have cameras that only turn on in case of movement on your property, hence triggering (I would hope) the legitimate interest provision in the GDPR.
It doesn’t matter whether you track people using a camera, ultrasound, WiFi signals or even manually by watching them from your window and keeping book about their coming and goings, what matters is that you process “their” data. So using a different technology to perform the surveillance doesn’t free you from privacy laws.
In Sweden RF based movement tracking using passive equipment should be perfectly fine as surveillance laws essentially only cover audio, images, and video. As long as it doesn't identify people, and doesn't violate expectations of privacy, I don't think there are any laws that could be applied.
Fixed cameras here must be placed so that they either don't show any non private property, areas the public is expected to traffic, or that all that might be imaged has given explicit permission.
Outside of this you need a permit, which is awarded rather sparingly, and which also includes the possibility of a mandated inspection of both cameras, control room, and any recorded material.
OP asked about filming/watching other peoples houses and the area sourrounding your own home though, hence my answer. You can film on your own property as you like, but if your surveillance system captures the movement of your neighbor it is possible to link the data to them as well (with high enough probability) so it becomes personal data. If you somehow manage to only capture potential trespassers and do that in a way that does not allow any linking to a specific individual (e.g. by only recording metadata) you can argue that it’s anonymous data, if you routinely capture all people moving through a given area you can’t though as it’s possible to attribute the data back to individuals using context information and statistics.
