That message means the ban was canceled by a share bot... and there's no easy way to detect that event in a script.
If you had the ability to read raw botnet traffic (there's a patch to do that here: http://barkerjr.net/irc/eggdrop/patches/ , but I've no idea how/if it works), there would be no problem detecting it, but you'd still have no easy way to detect a local unban.
You could do it on an unpatched eggdrop by keeping a copy of the banlist in a variable and comparing your copy to the actual list repeatedly.