Stealth Mechanics in Unity: Building NPCs that Hear and Alert Each Other Without Spaghetti Code

Implementing a believable stealth system is one of the biggest challenges for any indie developer. It is not just about the enemy seeing you; it is about how the environment reacts when you make a mistake. The real problem arises when we try to make NPCs communicate. If Guard A detects the player and has to manually call Guard B, C, and D, you end up with spaghetti code full of direct references that break your project when a single object is removed from the scene.

The Monolithic AI Trap

Traditionally, AI logic is written in massive scripts that try to handle everything: patrolling, vision, hearing, and combat. This is not scalable. At Noocturnal Games Studio, we believe the professional solution is total decoupling through an event-driven architecture.

Hearing Mechanics: SoundEmitter and SoundReceiver

For an enemy to hear your footsteps or a distraction, we need a propagation system. Instead of the player searching for all nearby enemies, the player emits a sound and interested parties react.

Implementation with Modular AI Behaviour Kit:

// The player emits a sound upon landing
SoundEmitter.EmitSound(radius: 15f);

// The NPC has a SoundReceiver listening for the event
public void OnSoundHeard(Vector3 position) {
aiController.ChangeState(AIState.Searching);
searchBehavior.StartSearch(position);
}

This approach allows any object (a rock, a grenade, the player itself) to generate sounds without knowing the AI exists. This is pure modularity.

Social Coordination: The Alert System

What happens when a guard spots the player? In a professional game, that guard must shout. But we do not want to call specific functions of other guards. We use the Alerter component.

• Alerter: When the PlayerDetector confirms vision, the Alerter performs an OverlapSphere to find other NPCs with the AlertReceiver component.
• AlertReceiver: Receives the threat position and processes it according to its own logic.

The Brain: AIController as the Conductor

To prevent an NPC from trying to patrol and chase at the same time, the AIController manages transitions. Upon receiving a social alert or a sound, the AIController shuts down the patrol module and activates search or chase.

Mastering stealth AI does not require thousands of lines of code, but an intelligent architecture. By using decoupled components, your game becomes robust, easy to debug, and most importantly, fun for the player.