Unless it's something truly unexpected, such as a spouse getting transferred or a parent suddenly falling ill and needing to be cared for, quitting a job isn't taken lightly. It's like a breakup. Except in extreme circumstances (like discovering your better half has a fetish you just can't abide), you hem and haw for a long time before cutting those strings.
In some cases, you may want to "save" your employee and it might even be possible. Maybe he or she is unhappy with a relatively minor issue but just didn't see a way to fix it or to verbalize discomfort in the right way. In other instances, you agree that it's a good parting but also need to make the transition as easy as possible--preferably by having the departing employee train or prepare training materials for a successor. Here are the signs you're about to get two weeks' notice and what to expect.
1. She/he clocks in and out at exactly the right time
Like the more obvious sign of an employee's coming in chronically late or leaving early, this can be a red flag. Of course, this is assuming that in the past she/he would occasionally come in early, work late, or volunteer for an extra project. If she/he's doing just the bare minimum, and that includes when she/he punches the clock, she/he no longer considers her job a priority.
2. She/he's not as friendly with her colleagues
Much like a personal breakup, doing "the fade" makes it easier for the person leaving. She/he knows she/he'll soon no longer be comrades with these people, and if she/he has genuine connections with them it's simpler for her to start distancing herself now. This all depends on how social she/he was to begin with, but an eagle-eyed manager should be able to pick up on it.
3. She/he just got a degree, license, or other certification...
...and has barely mentioned it to you. There are certainly cases when an employee is finishing a degree or decides to pursue a new accreditation while planning to stick with her position. However, if this undertaking is largely hush-hush, she/he's likely making herself look better for the job hunt. Otherwise, why would she/he be spending all that time, money, and effort just to keep a position she/he already has?
4. There have been major company changes lately
This one is tricky because when you have a lot of other stuff going on, you might not notice cues that a certain employee is unhappy with how it is all unfolding. Maybe there's been a lot of turnover lately (especially with management), or maybe there are new guidelines that some might view as strict. People are creatures of habit, and what can seem like an innocent change to one might be devastating to another. There's a reason turnover begets turnover.
5. She/he's just had a major, positive life event
From marriage to children, the big things in life that are filled with joy can also mean transition time for employees. While it's fair to ask about any changes that may accompany an employee's welcoming a child, don't overlook other big events such as her publishing a book, getting engaged, or trying out for the Olympics. With other things to focus on, she/he might think she/he doesn't have enough time for her current job.
Seeing a notice coming gives you an advantage, whether you want to try to persuade an employee to stay or get ready to transition in someone new. Keep your eyes open.