Thank you Baluncore for the answer.
https://novelda.com/ultra-low-power-sensor
with many high-tech boards easily available, a person with an EEE background can easily assemble a few Arduino-embedded systems concealed under some fixtures on the ceiling (e.g. clock, decorative..), monitoring the...
Ultra Wideband (UWB) RADAR systems discharge signals over a broad range of frequencies which is between 3GHz to 10GHz as compared to the conventional RADAR systems and remain extremely difficult to detect.
an excerpt from page 2 of...