That's why i mentioned the low-power Sleep mode. It has to be always doing SOMETHING, otherwise the nanites would suck the power from it and game over. It's clearly not always emitting the bubble that shuts off the nanites, as you'd have noticed that they always had electricity near them, but it can't be completely off, because it's just a USB drive plugged into something. No power = USB doesn't work. It's got computing power in there somewhere. Which is kinda what I said, before you said "no" and then restated it
I don't think there IS an area with no nanites at this point. If there's quadrillions of them, to the point where they're floating in the air being breathed in, they're in everything. After 15 years to spread and presumably replicate, no avoiding them. World-wide, as you'd get them into the water and jetstream, and they'd travel the world in short order.
But that'st he kind of technobabble I don't mind arguing, as you can reason your way around it one way or the other. You can talk about how they 'should' work together and play with it. It's what they stated that they DO in the first place that's dumb. How to disable them is just a magic spell to negate the other magic, whatever.