If you have a pi there, why not just just zero tier to that pi and monitor it's zero tier IP? You can do a very basic keep alive check with any monitoring system that can ping and then alert trough whatever script that monitoring system can call.
Or just have an old android phone running macrodoid and keeping that connected to usb power all the time.
If phone is no longer connected to power, send a text, call a number, write an email etc. You will need a sim card though. But one with very limited data can be had for very little money.
Prop up a phone so it can see the port LEDs of the switch and do a video call to that phone.
Then run around the house and plugin a laptop at each socket and see which port lights up.