A person who has an eye that looks away from the direction that the other eye is looking. For example, a person's left eye is looking forward while the right eye is looking to the right. Some people also have the ability to change which eye is the side scanner. Not to be confused with someone who is crosseyed or middle scanner.
The guy at the drive thru window was a side scanner. He was looking at us and the car behind us...at the same time!