Passive Wi-Fi monitoring service can tracks unique mobile users as they move through a mall, for example. It relies on Wi-Fi to do this, so it can track users who have their Wi-Fi turned on on their phones. Because it does not rely on GPS, it can track people indoors. Here is a demo version of dashboard for statistical service: Spotique. It is a-la Google Analytics, but for the real place and real visitors.
Snapshot illustrates registered mobile devices: