The original engineer states the RSSI thing from the start because it was an input to the meshing algorithm.

Using it for location of employees was the new part.

Most nodes in a radio based system will do rssi measurements to any other nodes that they need to communicate with directly, as part of deciding appropriate tx/rx amplification levels / when nodes are unavailable / etc. These functionalities are often easy to access to enable easier debugging, so it's a relatively straightforward change to start using them to scan other things on the same protocol etc.