Perceptive mobile network applications don't require people to have phones on them to be tracked, identified and monitored. Many passive sensing applications are being researched. Some require additional hardware, others only data analysis. But all of them rely on the relatively high densities of mm-wave RF being rolled out with 5G deployments (not to mention the THz frequencies being proposed for "6G").

Here are some of the applications listed in the paper "Enabling Joint Communication and Radar Sensing in Mobile Networks - A Survey" (https://arxiv.org/pdf/2006.07559.pdf)

- (Through-the-wall) localization and tracking;

- Human behavior recognition and fall detection;

- Monitoring of biomedical signals such as respiration patterns;

- Human presence detection and radio fence

- Extensive on-street and open space surveillance for security and safety