The core system in any stealth game is the guard AI, specifically visual acquisition and detection of the player.
In Sneak 'Em Up (working title), the guards start in a patrol or standing state, while constantly scanning objects that enter a circular trigger around them.
Once an item enters the green circle, it goes through a gauntlet of questions to filter out only the items the guard should respond to:
In this way, fast-moving objects that are close to the guard fill his Awareness exponentially faster than stationary targets that are far away.
The visual acquisition system is designed to not be specific to just the player, to allow guards to react appropriately to characters from different factions or wild animals, as well as objects like distractions or dead bodies. Systems like these will provide more chances for the player to use the world itself as a tool of chaos and destruction.
As awareness builds, the guard moves into first the Suspicious state, then the Alerted state. Taking--ahem--inspiration from the Metal Gear Solid series, the player receives visual feedback as to which state the guard is in. The Awareness system will be covered in more detail in the next Devlog.