He should have sensed someone coming, but he didn't, and is genuinely startled when someone says his name. A voice that was very familiar to him as he had heard several very similar verions of it throughout the war.

Obi-Wan looked up and then quickly jumped to his feet as he saw the Trooper in his armor. At first there was alarm in his eyes. The last time he'd seen clones... well. It hadn't been a very happy experience. But upon a closer look, he realized that he knew this particular clone.

"It can't be... Fives?"