What are some warning signs that one of my employees may be considering leaving the company?

Unexplained single day vacations in the middle of the week.

probably some incidents/conflicts should happened before or personal motivation

They begin repeatedly leaving work early, they add co-workers on LinkedIn, they clean out their desk, they constantly step away to take private phone calls.

lack of respect for people around them, lack of motivation, more time off at unusual times than usual

Signs of dissatisfaction with current job and or opportunities within the company

Your subordinates won’t listen to you. He/she won’t complete the task you assigned to him/her.

Distant, low energy calling out more often than before. Lack of enthusiasm. Whispering with conworkers and acting differently.

Disengagement, missing work more than usual

Lack of interest in their work.

If you are waiting for warning signs you missed the opportunity to have open communication and your team is already failing. Stop and fix this immediately.

Taking days off, working from home, running errands or taking sick days.

Reduction in engagement; reduction in productivity; reduction in enthusiasm.

If an employee suddenly stops fighting for things they previously cared a lot about, that is the clearest sign that they are preparing to leave, likely with no warning.

Taking a lot of sick days for interviews.

In my case, it comes from weekly career chats

Negative comments about work

Updated linkedin profile. long lunches. frequent cell phone calls with a quick dash to a private space. pushing out new work commitments.

Unhappiness. Past few coworkers I've had who left were not happy and simply left -- regardless if they put in their two weeks or not, to my knowledge none of them stayed that long.

Long lunch breaks. Lots of "errands" or "appointments".

They are over agreeable. They will talk to others about leaving or looking at other jobs.

If you have a good relationship with your employees, it shouldn't be a case of "warning signs." When I was looking at other jobs, my manager knew and helped me solve some of the underlying issues that led to me interviewing elsewhere.

lack of interaction with team members

Are they discontented? Are you paying them the current rate for their position/experience? Do you let them know they're appreciated? Do they come in late? Leave early? Grumble?

Taking time off, and saying it’s a doctors appointment. Grumbling

Work ethic and attendance

Start to become unproductive. Missing more time. Is not focussed.