Good question. I don't know if that kind of monitoring tool is available somewhere but you could make your own. Basically you just need to make ARP requests and this could be automated by a cron job (say once per day), then you compare changes with the previous results and you can see the changes (names in, names out) :gl:
It's the same thing that we do to produce lists of expiring domain names: comparing the newer zone file with the previous one and make a 'diff' to see the changes between the two.
Hope this helps. If you find that tool online, let us know... it's an interesting tool to have handy :tu: