If you have another outlet available in that outlet, this is a lot easier now thanks to a recent update. You can use theHubduino project to connect a $5 ESP8266 board to the other outlet via a simple USB wall-wart. New functionality with those devices has built in presence detection. So, when the power fails, the board fails to check-in with the hub on it's regular schedule and the driver changes the board to "Not Present". No special wiring necessary. No battery powered devices necessary. You will get a report to Hubitat within the time of your configurable timeout when the power fails for that circuit.