Clint's awareness of his surroundings was like a setting on a machine that was always on, but it certainly wasn't at the forefront. He was reading (passable) fiction and listening and tracking the movements of people around him, all at once, quite relaxed, which was simply how he operated in public after so many years. He never really focused on anyone until they crossed onto his radar to the point where they were too close to ignore.

Even so, this woman would not have tried to sneak up on him. As soon as he realized Natasha was there, he watched her approach. There was a mixture of gladness and wariness at the sight of her, because - well, because he was in some sort of weird dimensional world, and so was she, and that was just really odd. But the wariness left as soon as she was close enough to ascertain it definitely was her. Someone else could be fooled by a fake, or an illusion, but not Clint, not with her.

"Well, if it isn't the deadliest woman in the world," he said, lowering his book, reflecting his smile back at her. "I'm about two hours away from a panic attack, actually, but you seem cheerful enough."