While it certainly won't always be true, it probably is true most of the time.

Even if the employee was unhappy enough to go looking, changing jobs often comes with high transaction costs that most people won't want to deal with without a good reason.

Also, in the tech sector and some other areas, its not uncommon to wind up with an offer when you haven't put in the work to find another job. If you are well connected in your field, you may get offers, or at least invitations to interview, without actively looking for them.