All enemies guard areas limited by their fields of vision or other means of detection. Upon entering a guarded area, there is a chance that the enemy or enemies patrolling it will take notice. If your trespass is observed, a warning icon [!] will appear over the heads of any enemies alerted to your presence. These foes will then become hostile, taking up defensive positions or rushing forward to attack.
If your party leader comes into contact with an enemy, a battle will begin. Battles begin normally if the enemy was aware of the party’s presence. If the enemy was caught off guard, however, the party will be able to make a Preemptive Strike.
Certain enemies, like Zwerg Scandroids, have extremely narrow fields of vision and can easily be approached unnoticed. It is always advantageous to do so if possible, as initiating a Preemptive Strike forces enemies to enter battle with their chain gauges near the Stagger threshold.
Note that sometimes it’s impossible to gain a Preemptive Strike on an enemy.