Another fairly frequent sub-topic in the popsci press (besides correlation with sadism) in relation to narcissism is covert narcissism. To some extent that article confuses the covert aspect/presentation with the better studied "vulnerable" narcissism, but this can probably be forgiven because the DSM-IV itself wasn't terribly clear on this, although the DSM-5 makes more of an effort to distinguish between the two dimensions. (And of course calling it "covert" makes it sound much more ominous, so it makes better click-bait, even if you're mostly talking about "vulnerable" narcissism.)
Of course, any studies of anything covert are probably going to underestimate it if the subjects are really good at covering it, but nonetheless, just what percentage of narcissists are generally known to be of this kind? Furthermore, is there a different mixture of vulnerable/grandiose in the covert group vs the overt one? I'm hoping some empirical studies exist on this.