Scuttlebutt, bush telegraph, radio bemba, the rumour mill: this tends to be how we hear first that someone is leaving, or has been hired by another university. That would make sense for friends and close colleagues, but even for people at great distance, we end up getting third hand information about who is going where, and why. Unless you are close enough to someone to have seen their facebook updates, or in the department doing the hiring, you're unlikely to otherwise have a clue about who is moving departments next year.
Those of you who have braved the academic job market in recent years will be familiar with the various job wikis (many hosted at http://academicjobs.wikia.com/ - a lot scattered elsewhere on the web).These are examples of crowdsourcing, where many participants add to the pool of information. Of course, there is sometimes disinformation broadcast on the wiki - whether malicious or mistaken.The general cloak of secrecy allows a lot of disinformation to flourish, as incorrect assertions are rarely corrected - and almost never by anyone speaking with authority (the candidate in question, or a member of the search committee).
In the UK, where the period from application deadline to hire is often something like a month (with the job offer coming within 24 hours of the interview), there is not a long time in which people are required (or try) to keep a lid on the process. Not to mention that it's common to meet the other candidates at the interview - there is no expectation that one's job search is confidential, from the other applicants or anyone else. In North America and elsewhere, where a search can lurch on for months - that's a lot of time for dust to be kicked up and gossip to swirl among it. In Germany, it is common for people to advertise not only the new job they have accepted, but offers that they declined. I'm not sure that level of transparency would take off in the English-speaking world, but it's an interesting comparator to have when examining the manic paranoia that seems to accompany the job market in some places.
While some universities make announcements about new hires (in history, ads are placed in Perspectives), most don't. Big splash announcements tend to be limited to senior hires, and even then will sometimes only show up on institutional websites months after the fact - and that's at the hiring end. The departed department often retains faculty profiles on their site for those long gone, and not just those who took jobs elsewhere. I learned never to rely on a faculty directory for up to date information after the mortifying experience of writing to a scholar and receiving a very sweet note from his widow, telling me he had passed away years before...!
For philosophy, the Leiter Report does a good job of keeping the community up to date with moves. But I'm not aware of anything like that in history or other humanities. (It depends on first having a website that a critical mass of people in the field visit, and I'm not sure we do). This also offers a service for PhD students in throwing a little daylight into the shadows of placement rates, which are typically vague and anecdotal otherwise.
Now, you may be asking why you should care. Currently, few historians have a personal web page. When I search for someone online, I am lucky if the first result is their departmental website. But frequently the departmental site that comes up is the one of their grad school, in which they are still listed as a PhD student despite graduating five years ago, or the school where they spent a year as a VAP some time in the 1990s. And of course the listed email addresses are long since out of service. We all know the departmental pages that list someone's book as "forthcoming for 1998", and faculty info is no more likely to be current. How hard should it be to find other academics - perhaps to suggest a collaboration, or invite them to participate in a symposium? If you find you are hidden under the weeds and rocks of the google pond, make your profile shiny so we can see you!
Back to the transfers - unlike the Major League draft, there is no wholesale publicity of who is going where (although I bet that would lift viewership on historians.tv!). I was recently introduced to this section of Inside HigherEd which is a directory of people changing jobs. It seems to be little-used, and mostly by those being promoted within the same institution. Perhaps History News Network could start keeping track of historians on the move? They're the only site I can think of with the visitor volume that might be able to manage it.