Zone-level occupancy tracking is a critical technology for smart buildings and can be used for applications such as building energy management, surveillance, and security. Existing occupancy tracking techniques typically require installation of large number of occupancy monitoring sensors inside a building as well as an established network. In this study, in order to achieve occupancy tracking, we consider the use of WiFi probe requests that are continuously transmitted from WiFi enabled smart devices. To this end, WiFi Pineapple equipment are used for passively capturing ambient probe requests from WiFi devices such as smart phones and tablets, where no connectivity to a WiFi network is required. This information is then used to localize users within coarsely defined occupancy zones, and subsequently obtain occupancy count within each zone at different time scales. Our numerical results using WiFi data collected at FIU over several days show that utilization of WiFi probe requests can be a viable solution for zone-level occupancy tracking in smart buildings.