If "Anti-lost alert" is enabled, your ZeFit³ will notify you when the device paired with it (smartphone, tablet) will lose its Bluetooth connection (Bluetooth range: 10 meters).
You can enable this option in the Notifications section of your mobile application.
Once you are too far from your mobile device, the following icon 
will appear on your ZeFit³. As soon as you come close to your mobile device, your ZeFit³ will connect automatically.