Unaccepts should show up in your reputation history. I do remember seeing those notifications for a brief few months in the past, but not for a long time now (it is a rare occurence, so I have to wait for it to happen...).
Just now, I got greeted with another one of these mystery rep transactions:
Is this feature broken, or is something else at play?
Could it be that it has to do with the new (black) site banner/header/menu thing? It seems to me that the notification system in general is kinda broken/quirky since then. E.g. I seem to get the same notifications at least 3 or four times, sometimes unable to clear them at all, except by clicking on the "colored tab" notification counts in the profile page. I don't really mind that, but it might point at an issue?