I would suggest there is some interaction happening. Whether it be dismiss, deleting accept emails, or something, but I would describe that more as a blink.
I would suggest the only way they will really disappear and re-appear is Outlook 2oo3 sp2. In sp2 the ID will change with updates, etc. Either way the update will get sync'd and re-associated with the series (if that's a series). Anyway, have a read at this article and then get that user to show you what they mean. http://support.microsoft.com/default...B;EN-US;899919