Some alien is wondering where they lost their key-chain of doom. Now you know.
But is it a psychic thing that makes people go mad? Or does it issue a cloud of nanotech bugs that "clean" things up?
First post, yay.
I like the notion of nanobots because they are cool, relatively tangible sci-fi (I can imagine an apocalypse in the near future, where nanobots have been fully developed) and still, most players won't really have an idea about the limitations of nano technology.
In addition, they'd go well with the concept of fuckery an pushing players where they're not in control. You could consider if they just "clean up" microbes, bacteria, virii, or are harmful to larger organisms as well. Maybe they effectively prevent any process of decay in NO, leaving corpses to dry out and mumify. Maybe they start slowly, attacking your eyes, respiration system and upper dermal layer first.
You could describe how the player's characters get itchy after a while in NO, how their eyes and lungs burn, their noses become dry and irritated ... and then somebody starts coughing up blood (maybe after some physical exertion) as their alveoli start to burst. I guess you could build a countdown based on that.
Also: Consider if these nanobots got released simply due to apocalypse-whatever or on purpose. And by whom. And why.