Exclamation point
From the Metal Gear Wiki
The Metal Gear series commonly features the exclamation point during cases in which enemy sentries will discover the presence of the game protagonist. In these circumstances, the player is able to become aware of a danger and quickly hide, move from the area in which the designated sentry has become interested, or to equip a costume or the Cardboard Box.
[edit] In-game
In the original Metal Gear, a white shock bubble with an exclamation point meant that the only the guard team in that area would give chase in a attempt to finish off Snake. In other areas, the shock bubble was colored yellow and there would be two exclamation points inside of it. In this alert phase, enemies across different areas would be alerted and back-up would pour in from off-screen.
In Metal Gear 2, the exclamation appears in a small white bubble but the concept of a full red alert is finally realized, as Snake cannot disappear to the next screen or simply kill off the backup that comes his way. Instead he must hide or stay out of sight until the evasion phase finishes.
Beginning in Metal Gear Solid, the actual exclamation mark is depicted as a large, red "!" floating above an enemy upon their discovery of the opposition. Accompanying this symbol are the eyes of vigilant guards flashing with shock.
In Guns of the Patriots, when Snake first encounters Jonathan from the Rat Patrol, Jonathan's haircut (When seen from behind) looks like an exclamation point and the trademark Discovery sound effect is played.
The exclamation point is not the only piece of punctuation that appears above the opposition in the Metal Gear series. Whilst a colorless "!" means that the enemy has been alerted to a nearby presence through sound or movement, a colourless "?" is evidence that the enemy is curious or confused and will immediately resume his patrols. Actions such as placing distracting devices, such as empty magazines or books, upon the ground route of sentries will cause this behavior.
Other symbols can also appear over a sentry or an NPC. For example, white, floating Zs will trail off a character who is sleeping and small, yellow stars will circle a character who has been knocked out. A pink heart will also hang over a character if Snake gains a character's trust.
Shooting directly at the exclamation mark above enemy sentries will daze them for a few moments, indicated by white bubbles above the enemy's head.
