Today's job seekers don't have it easy, and new data indicates some hiring managers could be making things more difficult.

As many as 4 in 10 companies say they've posted a "fake job listing" this year, and 3 in 10 companies say they're currently advertising for a role that isn't real. That's according to a May survey of 649 hiring managers from Resume Builder, the career site.

Fake jobs, in this case, refer to online listings for roles the company isn't actively hiring for but wants to use to collect resumes, among other reasons. (Fake jobs that turn out to be scams are another issue entirely.)

It's also worth noting that while this proportion of companies say they've posted a fake job, that doesn't mean the same share of their listings are fake.

The strategy, frustrating as it is, isn't entirely new, says Stacie Haller, chief career advisor at Resume Builder.

For example, temp agencies "constantly need new talent and are always running new job ads because, if a client needs someone, they need [a candidate] who's already vetted," Haller tells CNBC Make It. In these situations, agencies may contact and interview candidates to keep their information on file until a real vacancy opens up.

Now, however, she's seeing that "more traditional corporate companies are doing this more."

Why companies post fake jobs

According to the survey, hiring managers are most likely to post fake openings for entry-level and mid-level roles.

They reported posting these listings across the internet, including their own company websites as well as online boards like LinkedIn, ZipRecruiter, Indeed and Glassdoor, among others.

That goes against the user policy of many of these job boards. A spokesperson from LinkedIn tells CNBC Make It that "we require recruiters to post jobs only if they intend to hire a candidate for the specific position indicated." ZipRecruiter responded to a request for comment with its job posting rules page that states submissions must reflect "a real and current job opening."

Still, a majority, 7 in 10, hiring managers say the practice of posting fake job listings is "morally acceptable."

Why spend the time and effort to advertise a job that's not actually open?

It all comes down to "reasons of appearance," Haller says.

Hiring managers say they believe advertising nonexistent openings has a positive impact on their revenue by making it appear like their company is growing faster than it is, according to the survey. They believe the practice increases employee morale by making over-extended staffers feel their workload will soon be alleviated. Others believe the move boosts productivity by making employees feel like they're replaceable and have to prove themselves against a potential newcomer.

Finally, hiring managers say they keep fake listings up in order to collect resumes to keep on file for later.

Regardless of the business's reasonings, Haller says the practice erodes trust in the hiring landscape and can leave job seekers feeling burned out.

"I can't say this is a good approach to hiring," she says: "None of it is acceptable."

Cold applying to job listings may be 'a last resort'

Knowing how many job listings might not be real doesn't exactly inspire confidence in online job boards. Indeed, "sending your resume to a job posting should be a last resort in terms of getting hired at that company," Haller says.

But that's not to say they're completely unhelpful. Think of online job ads as a starting point, Haller says.

First, check when the job was listed, Haller says. If it's been up for a few weeks, there's a better chance it's actually active than if it's several months old.

Second, check the company's website and see if the job is posted there. If so, try applying directly through the portal.

Finally, use the online post to find the hiring manager and send an email or LinkedIn message to express your interest, provide your resume and mention that you've applied through the company's official hiring channels.

Many job sites are also helpful to network and build professional connections, which can lead to job opportunities, Haller adds.

Some hope for job seekers

Despite the hiring practice, Haller says there's one small bright spot when recruiters post fake jobs: Of companies that listed a non-existent opening, 84% say they either always or sometimes contact applicants who submit their information. And of companies that contact applicants, 85% say they go ahead and interview candidates, even if it's not for an immediate opening.

"In many instances, those people do get looked at, and they can get hired in the future," Haller says, either once an opening is available, or perhaps if the company's next hire doesn't work out.

"The bottom line is, many times, if you are interested in the company and you're a good match, then when there's something available, they'll call you," she adds. "It just might not be today."

At that point, though, Haller says job seekers should consider whether they'd want to work for a company that posts jobs and recruits for openings in that way.

Of course, Haller says, "Honesty along the way would be a much better scenario."

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