When a company finds that a particular staff member "isn't a good fit for the company" they will add this person to an informal list that is traded through back-channels with other companies. At some future time such personnel might be directly approached by headhunters on behalf of inquiring companies. Of course the current employer overlooks the poaching activity, they want the member gone. The only clue the office gets is one day out of the blue that member shows up to work wearing full interview attire, and might soon after announce that they are moving on.
For the managerial tier and above, they frequently socialize and will have a sense of when it is time to move on, in accordance with societal expectations. Given that the managerial profession comprises a relatively smaller group in Japan, they can expect to land their next gig soon enough.
As for the rest, I've heard of plenty of tactics used to induce (eventual) voluntary departure, "black" or otherwise. I don't advocate for them myself, but the culture, market, and law is what it is..