I've been that employee. As my dad (a farmer) says, "the hired man wants a day off to go look for another job." There are many tell-tale signs that an employee is looking for another job, and the article is absolutely right that—until the they put in their notice—it is rarely too late to change their mind.
The thing is that while I was interviewing, I certainly wasn't telling the boss I was doing that. That's suicide. I certainly have lots of tells (and had lots of "dental work" like the two guys that quit before me). The CEO just didn't have his ear to the ground or talk to the rest of the staff. Especially the office admin: she knows everything.
If it's something other than money, chances are real good you won't be able to keep them.
But, if you grow fast enough ... this resolves itself.
If in your smaller organization really the best mobility is to jump ship, then you have to factor that already in hiring that you need someone who would like an organization that stays small, and beware of ambitious key people who'll not be good for you. But for startups it shouldn't be so, unless you fail.
Nobody at work noticed.