This is not a complete walkthrough, but these hints do cover the most common areas where players get stuck.  While the objectives in ZODIAC were specifically designed not to be "tricky," there's always the potential to be confused by something or to accidentally miss something.

--- ZODIAC Part 1: CIA Headquarters ---

Intro Mission: Hong Kong Briefing (no hints yet)
Mission 1: CIA Headquarters

--- ZODIAC Part 2: The Truth is Out There ---

Mission 2: Buenos Aires
Mission 3: Page Biotech
Mission 4: Cloning Labs (incomplete)

--- ZODIAC Part 3: End Game ---

Mission 5: Holloman Air Force Base (incomplete)
Mission 6: (secret location) (incomplete)