Yes, I believe it's happened in the past. Plenty of anecdotal evidence. Not sure about today. As someone said, it only takes a couple good 'ol boys and only a quiet word over a beer, not a massive, coordinated conspiracy.

Also as was mentioned, senior lifer types have a clear incentive to help stem the blood loss, even if nobody tells them to. Their comfy, reasonably lucrative gigs are on the line.

I have also heard of "retention bounties", but not sure if I believe it.