If you've been in management for any length of time, you probably know the obvious signs that you have a retention issue with an employee, that he or she might be exploring other job opportunities. They suddenly have a bunch of weekday "appointments." They take extra-long lunch breaks. They show up to work in a dress shirt and nicer shoes instead of the usual T-shirt and jeans.

It's never fun to find out a team member is unhappy working for you, especially if they're a rock star at your company. But the earlier you spot the more subtle signs, the more chances you have to up your employee engagement and get them to stay.

Here are four not-so-obvious clues that an employee is looking for a new gig, and what you can do about it:

1. Private calls during work hours

I once realized an employee was interviewing for another job when he would answer a call on his cellphone, then speak in a hushed voice while racing to the nearest empty office for privacy. No biggie if this happened once in awhile--everyone needs to take private calls during work hours every so often--but the fact that it happened several times in one week raised a red flag not just for me but for his cubicle mates, too. Sure enough, he put in his notice a couple of weeks later.

2. Disengaged work ethic

Sometimes changes in attitude and work ethic are caused by personal reasons, but other times they're because of dissatisfaction at the job. Be aware if an employee seems suddenly and significantly less enthusiastic; same with an employee who once had a great can-do attitude but now is just skating by and doing the bare minimum.

As a manager, you need to be tuned in to these ups and downs, and if it feels like productivity is taking a downward turn, there might be something more to it.

3. Social isolation

Think about it: If you had one foot out the door, the effort it takes to socialize and be friendly toward your co-workers probably no longer seems worth it. An employee who keeps to herself lately and just seems unengaged--especially if she used to be more social--might be unintentionally letting you know that something's not quite right anymore.

4. Increased LinkedIn activity

We all know LinkedIn is the social network for job seekers and professional networking, so watch out if an employee has a flurry of new updates and recommendations on his LinkedIn profile. If someone's profile suddenly gets a major makeover, it could signal something more.

"Please don't go."

If you don't want them to go, try to find out what's making them unhappy. It's never fun to hear about their grievances for the first time in exit interviews. While some employees might not want to open up to their managers, at least give it a shot and let them know you're open to changing their work situation.

If pay or title is the problem, can your budget to accommodate a reasonable raise? It's a lot cheaper to keep a deserving team member than having to hire and train a replacement. You might have to fight for it, especially if your company's holding on to outdated ideas about when and how promotions can be given out.

If your employee is unhappy with current responsibilities, help him or her seek out new opportunities within the company. You might have to advocate on your employee's behalf by talking to other department heads and paving the way.

It's unreasonable to think that your people will stay with you forever. But being able to identify subtle signs of dissatisfaction will at least give you a fighting chance to improve things--for both your team and your business.