Title should have explained it all. A simple Python script to monitor if a certain device has connected to the router via Wi-Fi, and send notifications accordingly. You can use this script for whatever purpose you want1I used it to monitor whether my parent has left home when I am “seemingly asleep”. , though probably you might not be able to find one like most of others.
To use this script, you need SSH access to the router, something in your LAN that is always running (in my case, a Raspberry Pi), and the list of MAC addresses to monitor. In this example, I am using an Asus RT-AC1200GU as the router. Other brand or make might need a different command.