In the ethernet port there is usually two LED's one green, and one orange, if you have not got anything connected then it will occasionally flash to show that its active, if you disable the ethernet in bios, it should stop it.
Unless you completely turn off your computer, then it will always continue to draw some supply, so to stop that you have to disconnect from the wall, before it will not draw anymore power.
In complete darkness, those little LED's can be bright, certainly noticable on mine.