My Dad was having to deal with finding out which plug receptacle is connected to which circuit at work. He came up with a simple idea of using NodeMCU's to help solve that problem. This is the result.

One NodeMCU acts as an access point and listens for UDP broadcasts from the other NodeMCU which connects to the access point. It sends out UDP packets 10 times a second. Once the client stops sending packets for 1 second, the onboard LED on the server turns off, indicating the correct circuit breaker has been found.