It will be one of your number channels within an MQTT Thing which is receiving the message off.
You can’t find the exact source with the log message alone - you’ll have to go through each number channel and check what it is subscribed to. Perhaps use MQTT Explorer at the same time to see what is being published to the broker.
You have a timestamp. Perhaps you can guess which of your MQTT devices is linked to a number channel, but might report an ‘off’ payload. Perhaps you can use mqtt-fx or similar monitor to see what MQTT messages were being sent at that time.