At this scene, Obi wan overhears the Seperatists plans. But one of the Seperatists leaders is a Sith Lord. Who is Count Dooku. In A New Hope , when the crew of the Millenium Falcon boarded the Death Star I. Darth Vader could sense that Obi-Wan is in the Ship.And he informs to Tarkin that Obi wan is in the Battle Station. Now why couldn't Tyranus sense Kenobi in Geonosis,Unlike Vader who sense Kenobi in the Death Star I? Dooku is a force sensitive because he was a rogue Jedi and a Sith Lord.
Anakin was Obi-Wan's padawan for many years. As Vader, he was still very familiar with Obi-Wan's Force presence, even after not being around him for decades:
I sense something... a presence I haven't felt since...
Dooku wasn't familiar with Obi-Wan's Force presence, so wouldn't have known what he felt like. That, and Obi-Wan was good at stealth and hiding his Force presence from other Force-sensitives. He just couldn't hide from his former padawan when in close proximity.
I will answer my own question if you don't mind.I saw a Youtube Comment by PolillaBenG that the comment is "What made Obi-Wan think he could still sneak around after seeing Count Dooku? I thought Jedi and Sith could sense each other's presences, so he should've at least suspected Dooku knew he was there." I saw majority of the replies of this comment that Dooku sensed Obi Wan in Geonosis which I started to believe that it make sense. If you're going to ask me why?Dooku knew that Obi Wan was spying him. But he did that on purpose, by luring him into a trap. As you can see in the film, that he was captured so that Dooku would plant a seed to the Republic in order the Clone Wars would have begun as Sidious and Tyranus planned.
Anakin and Obi-wan are connected by the force. This was exploited by Palpatine and Count Dooku to try and turn Yoda to the dark side. The reason that Count Dooku doesn't sense Obi-wan is that he doesn't know him. Think of it like seeing someone's face, but instead of actually seeing their face you can only see the back of their head since you know what they look like you can easily deduce whos face that is. I believe it's like that. But like the force. Also, in Attack of the Clones Count Dooku actually senses Yoda entering and recognizes Yoda's force signature.