I kind of brought this up jokingly in the chapter 421 thread, but there's the possibility that Hattori could be a spy for Aokiji (brilliant, a spy within an organization of spies/assassins) whose job would have been to keep Aokiji apprised of the latest happenings within the secretive CP9 organization, and how close Spandam and the Five Elder Stars are to obtaining the plans for Pluton. If true, then it would make sense how Aokiji, and thusly the Buster Call personnel knew that the mugiwaras are in Eneas Lobby, and that they numbered around 60 people (that of course includes the FF and Galley-La people).
I'd still find it funny though if it turns out that Hattori is actually a spy for Aokiji, not because I find the idea implausible (how would that be implausible when you consider that in the Alabasta arc there was a seashell wiedling otter flying around on a gattling-gun-toting vulture drawing portraits of people?), but because I find the idea of a talking spy pigeon to be rather humorous.