Use settings in the EnemyController script to adjust the visibility range. During the game, if a character enters this area, the enemy will see him.
If you set the Use States checkbox the enemy will use Warning and Find state during the game.
If the enemy sees or/and hears (read here about the character's noise) a character for a while or the character has shot him a few times, the enemy's warning state is activated and he will look for him.
If the enemy has found the character the attack state will be activated.
If the enemy doesn't see or hear the character, he looks for him for а while again. And after that returns to a waypoint.
If this feature isn't active the enemy will do the following:
If the enemy sees or hears the character, he immediately attacks him.
If not, he immediately returns to a waypoint.