You might want to take a look at Big Clive's video about the radar motion detectors in Chinese E27 socket LED bulbs [0], which have become cheaper even back in mid-2016 than good old PIR motion detectors. The latter are basically a castrated Flir-style camera, the former closer to this.

[0]: https://www.youtube.com/watch?v=FgdXRLjYkc4

Not through walls. Radio penetrates a lot more than heat.

Thermal doesn't work through/around walls.