Weekend discussion: Is there a 'hidden job market'?

Over at Science Careers, Beryl Lieff Benderly has a review of a book titled "Networking for Nerds", (by Alaina Levine) with this interesting paragraph (emphasis mine): 

Such a story of career serendipity is not rare, she writes. In fact, it illustrates a basic truth about the job market, especially outside of academe: “Most jobs are NOT advertised and neither are most opportunities that have the potential to be career game-changers, such as invitations to meet with someone, serve on a committee, pursue a leadership role, or apply for an award,” Levine writes in the book. When job ads do appear, furthermore, “often times, the committees already have people in mind whom they want to invite to apply or have promised the job to someone under the table.” While in many work sectors, such as academe and government, “jobs are legally obligated to be advertised,” that doesn’t mean they are necessarily open. Instead, often “there’s already a short list of candidates identified in the search committee’s head,” she tells Science Careers in an interview. This practice is so common that in Washington, D.C., government circles, there’s even a word for it: Such jobs are said to be “wired.” 

But alongside the “not-really-open” openings that appear in many job ads is what Levine calls in her book the “Hidden Platter of Opportunities.” This consists of all the needs and problems that people in every kind of organization constantly face, but that they have not yet announced publicly or reduced to job descriptions—or, as the Skutlartz case illustrates, even begun to consider in those terms. “The idea [of] a hidden job market is often mystifying to a lot of engineers and scientists. They’re so used to seeing ads,” she says in the interview. “In business schools, on day one, they teach you about networking. … They teach you how to honorably promote yourself.” But science or math programs don’t do that. During her math studies, she continues, she never heard a professor or mentor say, “If you want to advance in your career, you have to network.” 

There are a number of claims here, all of which are worth discussing:

• Most jobs are not advertised
• Most "career-changing opportunities" are not advertised, either, and come through networking
• Short lists are already in place for many positions
• Many organizations have "not-really-open" opportunities

Bluntly put, I think this is mostly non-falsifiable baloney. Think about your workplace - how many job openings are there advertised? Do you think there are 100% more job openings (the mathematical threshold needed for 'most') that are not advertised? 

I do agree that, for many mid-level positions, organizational insiders likely know where "we could use somebody here", and what that person might look like. But these are likely for positions where 1) people have requisite job skills already established at the entry level, 2) these positions are 'coordinator'-type roles where good candidates have two sets of skills and can talk to each party, i.e. a sales position that coordinates between the technical folks and the customers. These are not positions that are open to inexperienced folks. Also, I do not imagine these positions represent the bulk of available job openings. 

Let me be clear: I think "networking" is important, especially when people have an open mind to what each party needs. That said, I think that the concept of a "hidden job market" is fundamentally unproven, likely unscientific and may be an unnecessary source of distraction to job seekers.

Readers, I have my prejudices and you have yours - how I am wrong?

UPDATE: OK, one more question: how would you go about proving or disproving the existence of a 'hidden job market.'?
UPDATE 2 (10:36 AM, 20150928): Perhaps the issue is that there are three job markets?, the "open job market", the "obscure job market", and the "hidden job market." Reports are unclear about the "hidden Yeti market", though.