1. A tracking device, comprising: a telecommunication transceiver; a WiFi receiver; and a microcontroller, configured to operate the telecommunication transceiver to transmit WiFi information to a server during a data-collection period for behavior analysis of a tracked object equipped with the tracking device and for safe-zone demarcation of the tracking device, wherein: the WiFi information indicates WiFi access points detected by the WiFi receiver; the safe-zone demarcation of the tracking device is adaptive to habitual behaviors, obtained from the behavior analysis, of the tracked object; the tracking device is regarded as being located within a safe zone when the tracking device detects any trustworthy WiFi access points approved by the server for the current time slot in accordance with the behavior analysis; the data-collection period contains N days, where N is a number and each day of the N days is divided into time slots; the WiFi information collected by the tracking device in the same time slot between the N days is transmitted to the server for a correlation analysis to estimate confidence levels of the WiFi access points for each time slot of a day; and each time slot of a day corresponds to a WiFi confidence threshold to be compared with the confidence levels of the WiFi access points and thereby the trustworthy WiFi access points in each time slot of a day are obtained.