Scan every ghost, creature, or paranormal phenomenon (like Black Slime). Whenever you encounter a creature for the first time, scan it before engaging in combat. Some creatures look similar, so scan frequently and scan often! This can be unlocked by replaying missions you've already completed.
NOTE: To tell whether or not you have already scanned a ghost/other paranormal entity, pay close attention to your PKE meter. If a ghost has already been scanned the wings of the PKE meter will not fully detect it.