I agree that it's unrealistic to expect that the disappearance of a former President of the United Federation of Planets will not eventually be detected and brought to the attention of the general public.
well, one of Australia's Prime Minister did disappear off in the sea while he when swimming, and he wasn't even an ex-PM then.
Yeah, I know. And his disappearance was noticed almost immediately, wasn't it?

I'm not arguing that it's realistic to necessarily expect that word of Zife's assassination will get out -- though it may be, given that that reporter from Trill was able to put it together -- but the idea that no one would ever notice that a former president was just not around anymore is kinda silly.