https://www.xfinity.com/support/articles/wifi-motion-faqs
Monitoring/logging the the dampening of a WiFi signal between fixed WiFi items and WiFi router in your house assumes a person blocking the signal temporarily and can be used as an input for an early warning/notification system.
Same as a sudden change in infrared (heat) can be used to trigger a light to switch on. Like how a PIR sensor switch works.
There will be times the PIR will be triggered and the light goes on, but there's no person to be found. So it is by no means a verification that what triggered the switch was in fact a person.