Metal Gear Solid: Peace Walker/Gameplay

Stealth Phase
This is the default state of infiltration. Enemy soldiers are not aware of your presence and are following discrete patrol and guard patterns. They will check for anything that catches their attention, such as footsteps, or the player entering their line of sight. If something makes an enemy soldier suspicious, he will investigate the source. If he doesn't see the player at this time, he will resume his patrol route. If the player enters an enemy's field of view, the enemy will be startled for about a second before the Alert Phase begins. During this "grace period," the player may attempt to neutralize that enemy to stop the Alert Phase.

Alert Phase
The enemy has seen the player and has alerted his comrades to your location. In the alert phase, enemies will take cover and attempt to assault your position. Alert is also represented by a countdown displayed on the upper lefthand corner of the screen. It will remain at 100 as long as even one enemy soldier has line-of-sight on you. Only after losing the line-of-sight from all enemy soldiers on the map does the Alert Phase count down. When it reaches zero, the enemy moves into the Evasion Phase.

Evasion Phase
During the Evasion Phase, the enemy has a "last known location" for the player and are focusing their search there. Like with the Alert Phase, there is a countdown for Evasion. If the enemy re-establishes line-of-sight with you during this phase, the Alert Phase resumes with a count of 100. Once Evasion counts down to zero, the enemy enters the Caution Phase.

Caution Phase
During the Caution Phase, the enemy gives up on searching the "last known location" and they proceed to methodically search every corner of the map for the player until the Caution countdown reaches zero. When that happens, you return to the Stealth Phase and all enemies will return to their initial patrol routes while any backup called in will leave the map.