I suspect you won't find anything unless you click on the ad and follow prompts to install something-or-other.
We moved to a different ad partner on the 11th July, but I guess there's a certain pool of ad marketplaces that sit behind most ad-serving, so once you've been targeted for whatever...
It could be a rogue ad that's triggering this fake warning.
The problem with when this happens is, due to ad targeting, very few people may get the same. I use NSC a lot, always on a Mac, and I've never seen it.
If you get it again and are able to view page source and try to inspect on-page...