Mains hum is by far the most powerful source of electromagnetic radiation in most indoor places. And the device doesn't have to be connected to the mains to pick up EMI from it - turn up the gain on any battery-powered preamp and you'll hear it.
> Digital recordings almost always have mains hum on them, either because the device was plugged in to the mains or because it inducts it off nearby cables, lights and appliances in a room.