Start flipping breakers until you find out what circuit is using it, and work it out from there.
120W seems a bit high, but it's not firmly in the realm of "things I'd spend too much time caring about."
Everything in your house is going to be using some amount of standby current. The fridge, the furnace, the computers, the TV, the smoke detectors, the oven, the microwave...
It's also possible that the zero calibration on the current sensor is a bit off - you could cut all the breakers and see if it still shows a reading.