The title might have been vague so let me clarify. I have see this in many movies and television series over the years, even cartoons. Let me create a scenario since I can't visually remember one to link to it.
Say Tom is looking through a telescope for Pam. He is looking and gradually moving vertically to the left.. he passes Pam.. 'he realizes' and sharply jerks back to Pam bringing her into view.
Not to answer my own question, but I get that this is trying to show that the person is actually looking for someone and might be in such a hurry that they pass them quickly or overlook them etc, but after possibly decades of its usage hasn't it become extremely obvious to viewers? Why is it still such a common trend?