By Anne Fisher
August 16, 2013

FORTUNE — Dear Annie: I saw your post a few weeks ago about how to tell whether managers are thinking of moving on, but what are the signs that employees are planning to quit? One of my best people is a lot less enthusiastic lately than he used to be. He’s also been absent quite a bit, to the point where he missed a client deadline, which is totally unlike him. I’ve tried speaking with him and asking whether everything is okay and if there is anything I can do to help, to no avail. I don’t have much experience as a team leader, so I could use some advice on what to do next. —Concerned in Cleveland

Dear Concerned: No doubt, you aren’t the only one worried about losing star employees. About 2 million people quit their jobs in June, the Bureau of Labor Statistics reported last week, and that number seems likely to rise as the economy slowly but surely improves. “But people rarely leave without warning,” says Paul McDonald, senior executive director of global staffing firm Robert Half International. “Employers tell us they often see five signs that someone is getting ready to quit.”

Unfortunately, you’ve already noticed three of them: A drop in enthusiasm for, and engagement in, the work; more frequent absences; and a sudden increase in errors, like that missed deadline.

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Of course, any or all of these could be explained by something other than a job hunt (a personal problem the employee doesn’t want to tell you about, for instance), so how about the other two? “Watch out for less frequent interaction with the boss, as if the employee is purposely avoiding you,” says McDonald. “Another red flag is coming to work in more formal attire than usual,” which practically screams “job interview.”

If you have team members aged 18 to 34, they may well be carrying on a job hunt right in front of you. Almost half (48%) of Millennials, in a new survey by Robert Half division Accountemps, say they have no qualms about searching job boards, taking calls from recruiters or prospective employers, or polishing their resumes at the office, rather than waiting until they get home.

“It’s interesting that the willingness to use company resources for a job search declines as people get older,” McDonald observes. Only 13% of workers aged 35 to 44, and 9% of those 45 to 54, say they would job hunt at work, he notes, adding, “You have to figure that younger workers lack the experience to know what’s appropriate, or what the negative consequences could be. They’re also less loyal to employers in general.”

What can you do now to keep your star? Even though you’ve already tried asking him what’s bugging him, McDonald suggests you try again. “Retaining top performers is one of the most important things managers do,” he says. “A big part of it is communication, in particular finding out what motivates this person and what would keep him engaged. It might be a promotion, or flextime, or steering the employee toward projects in other parts of the company that would expand their knowledge and keep them challenged.”

While you’re at this, try to find out how the employee sees his career long-term. Top performers often get restless when they feel stuck in their current role, with no clear path to something better, so “talk about what opportunities you see ahead for this person, either in your own area or elsewhere in the company,” McDonald adds.

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If you get in the habit of having these conversations with your most valued people on a regular basis, McDonald says, you’ll have less reason to wonder if they’re job hunting in the future.

“As managers, we spend most of our time on problems, including underperforming employees, and on getting our own work done. Budgeting time for high performers becomes almost an afterthought,” he observes. “What works a lot better is flipping that order on its head, putting top employees first, then our own work, then problem employees.” Easier said than done but, as holding on to stars gets harder, it’s worth a try.

Talkback: If you’re thinking of changing jobs, what would convince you to stay with your current company? If you’re a manager, how do you keep employees engaged? Leave a comment below.

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