Almost every guard in the game, excluding the reinforcement guards, have a tag around their necks. There are about 300 in total, from each of the different difficultly levels (except European extreme), and can be collected by the player.

To do this, simply walk up behind a guard without him seeing you, and raise your gun in "first person mode", pointing it at the back of his head. He will raise his hands and surrender. Now hold L1 to lock on to him, and release the "first person mode" button. You can now walk round in front of him. Now, go back into first person, and aim your gun at either the guard's head, or private areas, and he will surrender his tag.

In some cases, they are unwilling to give up so easily. When this happens, shoot them in the leg with a gun (not the sleeping darts) and they will become more willing to help.

Tag rewards -

Collecting tags not only demonstrates your prowess, but also gains you rewards. By collecting tags you will first receive a bandana, giving you infinite ammo when the item is equipped. Secondly, you receive stealth gear, allowing you to pass by cameras and guards undetected. Later rewards include wigs that allow you to hold your breath forever, and grip for infinite time.

Tactics -

The key to the game is patience. If you take your time, and work out where guards, cameras and over obstacles are, then you can think out a route to get round them. If you rush in, without thinking, you could easily run into a large number of aggressive guards.

Look to your surroundings for help, there are often objects that can be used to aid your progress. Pipes venting steam into guards, fire extinguishers spraying foam near laser gates to show active sensors and more.

Take out enemy radios to prevent backup being called. When an enemy spots you, they will reach for their radio and call for help. If you've already shot their radio, then they will have to face you alone, increasing your survival chances.