At this scene, Obi-Wan overhears the Separatists 'plans. But one of the Separatist leaders is a Sith Lord, Count Dooku.
In A New Hope, when the crew of the Millennium Falcon boarded the Death Star I, Darth Vader could sense that Obi-Wan was on the ship, and informed Tarkin that Obi-Wan was on the battle station. Now why couldn't Tyranus sense Kenobi on Geonosis, unlike Vader who sensed Kenobi on the Death Star I? Dooku is force-sensitive because he was a rogue Jedi and a Sith Lord.