I'm going to sound like an idiot since I didn't bother to note the source. It seems that Intel, at least, has built in the ability for their chipsets to periodically wake from shutdown and check for updates. The newer chips and chipsets are never truly off, they never were anyway, but now the OS can use that to do updates and phone home.
The way I read the article led me to believe I'd have to shut down the power supply to be truly off. Otherwise there are several supported levels of "off".