Early Alert that an Employee is Going to Bail on You

Retaining (good) employees rather than hiring new ones is crucial on so many levels, so finding out that an excellent employee was disgruntled enough to leave you only after he put himself out on the market, is simply frustrating.
You can usually tell months in advance that your guy is having issues, but just in case it slips the radar, wouldn't it still be great to get an advanced warning? and what better way to do it than use the lack of privacy wonderfully embraced by all of us social media followers. This time - Linkedin.
How to do it? move to DEFCON 2 whenever someone has updated their Linkedin profiles. DEFCON 4 if they updated their specialties. DEFCON 5 if they start sending and (reciprocally) receiving recommendations.
Take them for a talk, see what's itching, win them back. It's foolproof...