Also, the character is allowed to open/close doors and even lockpick doors in this state. a person's bedroom or the legions armory). When a character is in sneak mode, it allows the character to move into restricted areas (i.e. There does not appear to be any benefit to raising sneak above 5. Your sneak skill also reduces the AP cost of entering sneak in combat, becoming 1 AP at level 5 (which is particularly important for making full use of the Guerrilla talent). The higher the character's sneak, the smaller the LOS cone of enemies.

You're perfectly able to sneak without any levels in the ability adding ranks simply makes sneaking easier.