Recognizing the shortcomings of traditional PIR and ultrasonic sensors used in alarm systems, security researcher Connected Ape decided to explore other options. He discovered this patent for a mesh system of RF nodes, designed to detect the presence of humans based on changes in the baseline signal strength between nodes. While such commercial systems are more costly, their advantage is the ability to pass through walls, furniture and other obstructions. This principle is implemented using signals in the 2.4 GHz range, and is found in systems such as those from Xandem which use the TI CC2540 RF SoC.
You can find more information and links in the post at Cybergibbon’s blog.