John Nottingham says he was planning to hire a new design manager eventually. But when he heard a talented fellow alumnus of his design school was looking for a job, he wasted no time: He created an opening and hired the man right away.

Under normal circumstances, Mr. Nottingham, co-president of product design and engineering firm Nottingham Spirk, might have posted the opening on the Cleveland-based company's website or LinkedIn page. But in this case, he says, he couldn't afford to wait. "Someone good was available, and we just grabbed him," Mr. Nottingham says.

"Even when you're happy in your job, you should always be making new connections," says executive-career consultant Debra Feldman. Getty Images/PhotoAlto

How to Stand Out It may be tough for outsiders to crack the hidden job market, but job seekers can find ways to get on the inside track. Some strategies: Make coffee and lunch dates . Reach out regularly to managers from other departments for lunch or coffee, even if you're not in the market for a new position, advises executive-career consultant Debra Feldman. When it's time to make a move–or if your department unexpectedly downsizes–you'll already be on their radar. "Even when you're happy in your job, you should always be making new connections," Ms. Feldman says.

. Reach out regularly to managers from other departments for lunch or coffee, even if you're not in the market for a new position, advises executive-career consultant Debra Feldman. When it's time to make a move–or if your department unexpectedly downsizes–you'll already be on their radar. "Even when you're happy in your job, you should always be making new connections," Ms. Feldman says. Put your goals in writing . Duncan Mathison, co-author of "Unlock the Hidden Job Market," recommends creating a list of the jobs that might best utilize your skills, the kinds of companies you'd excel at, and the type of manager you prefer. This "targeted opportunity profile" will help identify the organizations and people that are key to getting a desired job, he says.

. Duncan Mathison, co-author of "Unlock the Hidden Job Market," recommends creating a list of the jobs that might best utilize your skills, the kinds of companies you'd excel at, and the type of manager you prefer. This "targeted opportunity profile" will help identify the organizations and people that are key to getting a desired job, he says. Get candid with the boss . In line for a promotion? The only sure way to find out is by checking in with your manager, Ms. Feldman says. You could be waiting for the position above you to open up – only to discover that management is eyeing someone else for that spot.

. In line for a promotion? The only sure way to find out is by checking in with your manager, Ms. Feldman says. You could be waiting for the position above you to open up – only to discover that management is eyeing someone else for that spot. Seek out hiring managers. Attend events likely to attract key contacts in your field, such as panel discussions about topical issues and popular happy hours. "A lot of people make the mistake of just showing up at a networking event and meeting with other unemployed people," says Mr. Mathison.

With the labor market remaining weak, such back-channel methods are becoming the rule, not the exception, when companies hire. Many open jobs are never advertised at all, or are posted only after a leading candidate—an internal applicant or someone else with an inside track—has been identified. Sometimes, as in Mr. Nottingham's case, a hiring manager creates a new position ahead of schedule to accommodate a favored prospect.

While this "hidden" job market frustrates applicants, companies point out that it is perfectly legal to hire without advertising a job or to advertise one almost certain to be filled by an insider. They say internal hires generally perform better than external ones, at least initially, as research has shown.

Duncan Mathison, an outplacement executive and co-author of the 2009 book "Unlock the Hidden Job Market," concedes that anything hidden is difficult to measure but, by parsing labor statistics and recruiting surveys, he calculates that around 50% of positions are currently filled on an informal basis.

Even though federal labor rules don't require employers to post openings, human-resources departments at many companies require them to be listed on a job board or career site for some period, says Debra Feldman, an executive career consultant based in Greenwich, Conn. Such postings are meant to make hiring fair and transparent, and may help to protect employers from discrimination lawsuits or audits by the Equal Employment Opportunity Commission.

But hiring managers frequently sidestep personnel requirements, forcing HR representatives to step in and "re-educate managers about the reasons for the policies," says Lynn Hutson, director of talent acquisition at Brookdale Senior Living Inc. in Brentwood, Tenn. "We tell them we have resources to help them, and we can find them a bigger pool to draw from."

Learn More & Interact Have a question for Duncan Mathison about finding unlisted opportunities? Tweet it using #hiddenjobs and look for answers at WSJ.com/AtWork.

At a previous job, Ms. Hutson says she sometimes warned hiring managers that the organization could lose federal grant money if they didn't recruit widely, and added that the established recruitment process occasionally turned up better prospects, especially when a manager's preferred candidate proved to be a bad fit.

Not all HR departments are willing to fight that fight, and not all managers want to sift through a pile of strangers' résumés. Nottingham Spirk's Mr. Nottingham says his 40-year-old firm has built up a reliable workforce mainly through word-of-mouth hiring. The company often recruits on the campus of Case Western Reserve University, where its offices are located. "We can go to a professor and say, 'Who's your best student?' " he says.

Some HR officers don't mind being bypassed. Tim Sackett, a former staffing director at Applebee's International Inc. who often had hundreds of openings to fill, says he was relieved when hiring managers chose not to involve him in recruiting.

The size of the so-called hidden job market fluctuates with the broader economy, according to Mr. Mathison. When the talent market is tight, companies must advertise to fill key positions, making more open jobs public. In a soft economy, however, companies do more "opportunity hiring," creating jobs specifically to lure or keep promising individuals, he says.

Using data from the Bureau of Labor Statistics, Mr. Mathison calculated the difference between the number of jobs that employers said they hoped to fill in the following month and the number of employees they actually hired. When a company hired more employees than it officially estimated, he surmised that they were filling unadvertised positions. To that he added another 30% of all jobs filled to account for the number of advertised positions that ended up going to inside candidates.

"Managers are still looking for people and keeping track of the best talent," even when hiring is frozen; when jobs do open, companies already have a handy pool of candidates, Mr. Mathison says.

Fair or not, the practice irritates many job seekers, who feel shut out of companies and often don't know they are applying for phantom positions.

"You never get a fair opportunity to show what you have to offer," says Jo Ann Bullard, an HR specialist who was laid off in April by Orc Software. She says she has since applied for more than 500 jobs and has interviewed for several of them, only to later learn from HR contacts that those companies preferred to promote insiders.

It can be nearly impossible to know whether a posted job is real, Ms. Feldman says. She recommends staying current with people who work in a given company: they will be among the first to know when someone is being transferred to another division, or when a firm is building a new team for a product launch.

Sometimes, it is obvious when a listing refers to an all-but-filled position. Take, for instance, a recent posting for a head football coach at the University of Wisconsin-Madison, a Division I Big Ten school.

"There are probably four or five people in the country who would be considered for that job, and I doubt any one of them will hear about the job from the ad," says Mr. Sackett, now president of HRU Technical Resources, an information-technology staffing agency in Lansing, Mich.

The university, which has a policy of posting ads for all openings, admits the ad was "a formality." The eventual hire, former Utah State University head coach Gary Andersen, was recruited by the school's athletic director, says spokeswoman Amy Toburen.

Write to Lauren Weber at lauren.weber@wsj.com and Leslie Kwoh at leslie.kwoh@wsj.com