Security is protection from, or resilience against, potential harm. Beneficiaries of security may be persons and social groups, objects and institutions, ecosystems, or any other entity or phenomenon vulnerable to unwanted change.

https://github.com/jokob-sk/NetAlertX
Network intruder and presence detector. Scans for devices connected to your network and alerts you if new and unknown devices are found.
https://github.com/leiweibau/Pi.Alert
Scan the devices connected to your WIFI / LAN and alert you the connection of unknown devices. It also warns if a "always connected" device disconnects. In addition, it is possible to check web services for availability. For this purpose HTTP status codes and the response time of the service are evaluated.
https://github.com/netalertx/NetAlertX
Centralized network visibility and continuous asset discovery. Monitor devices, detect change, and stay aware across distributed networks.