Which product would make the most sense to use, if I want to monitor a closed circuit in a hotel room which has a power card switch? Ideally in combination with the ESP8266 to send the data. If the card is removed, the circuit is no longer closed and doesn’t provide power (240V Max. 15A AC).
So you may not need to complicate it with hardware. You can tell the ESP module to broadcast on interval that it is alive. If you can power the ESP off of the same line that is being monitored it would only broadcast when its powered.