The Wi-Fi Theft Alert function does not block access from unknown devices or hackers on your network, but only informs you which devices are currently connected to your network.
The reason there is a device connected to your wi-fi could be:
- Because it is not password protected
- Someone has discovered the password
- Your network has been hacked, in this case, it is recommended to change the password and activate other extra security settings, available according to each router model.