Originally Posted by
Old Manus
The reason you can tell when electrical things are on is because you can hear them
This. Electrical devices often emit various forms of noise simply from being turned on, regardless of what's supposed to come out of the speakers of the device. Old CRTs have a very distinct noise at somewhere between 14 and 16 kHz, depending on which refresh rate it was set to. Many other devices have some degree of noise coming from voltage transformers, or other components that you'd think would be entirely quiet just because they have no moving parts.
There's nothing magical about this, and if we had a better sense of hearing, like a cat's, we'd probably hear even more strange noises from our devices.
Come back when you can reliably read the future or something equally impressive :p.