An exceptionally low U.S. unemployment rate of 3.9 percent is making finding and holding on to workers one of the thorniest issues facing companies both large and small. So, many recruiters are turning to a once overlooked source of talent: Workers who left the company for greener pastures. Many companies are cranking up their alumni programs and reaching out more proactively to former colleagues with the hope of wooing some of them back.

And as George Anders, senior editor-at-large at LinkedIn, reports, such job-market boomerangs are becoming an increasingly popular way for people to boost their careers. He cites a new study by LinkedIn of 76 million job actions over the past decade which shows that the national rate of workplace boomerangs has more than doubled, hitting nearly 3 percent in the first half of 2018. At some companies like Wells Fargo and Cisco, the boomerang rate runs far higher.

Yiting Zhang, a senior LinkedIn analyst who oversaw the study, shows that for long-established technical fields such as aerospace, computer networking, energy, and telecom, rates approaching 5 percent are common. Even government, hospitality, and higher education "clock in around 4 percent."

In fast-growing new fields such as Internet technology, however, rehire rates barely reach 2 percent.

Zhang's analysis also found a difference between the rate of men and women boomerangers. In the past year, 3.08 percent of male job switchers chose to return to a prior employer, compared with just 2.66 percent of female job changers.

LinkedIn's data also showed differences across industries in the length of time it takes for people to boomerang back to their old companies, with 33 months being the overall average. "Faster turnarounds are common in fields such as newspaper (23 months), internet (25), and higher education (27). But the journey home takes longer in areas such as law enforcement (39 months), airlines (41) and semiconductors (58)," says Anders.

For employees, Anders offers a list of suggestions that will position them well for a potential future boomerang back to their company, should they seek to make such a move someday. He advises leaving your company on good terms, keeping in touch with them after you leave, and upgrading your skill-set. He also suggests "bringing an open-minded curiosity to the ways your former company has evolved."

And what should companies do to appeal to potential boomerangers? "Make an employee's last day at work compassionate," suggests Macy Andrews, head of Cisco's talent branding team. Build or enhance an alumni program that fosters a sense of community among departed employees and provides a channel for companies to update them on recent developments. And design more tactical programs aimed at particular sub-segments of the alumni population, such as those who left their jobs because of family obligations, but who are now prepared to return.