SmellSource.cs

using UnityEngine;

public class SmellSource : MonoBehaviour
{
    // Here you can add your smell types
    public enum SmellType { Treasure }
    
    // Set a default type
    public SmellType smellType = SmellType.Treasure;
    
    // In what radius can the smell be detected
    public float smellRange = 50f;
    
    // This calculates how strong the smell is at any position
    public float getSmellIntensityAt(Vector3 detector_position)
    {
        float distance = Vector3.Distance(transform.position, detector_position);

        // If too far away, no smell
        if (distance > smellRange)
            return 0f;

        // Smell gets weaker with distance
        // Close: 1.0, Far: 0.1
        float intensity = 1f / (1f + distance * 0.1f);

        return intensity;
    }
}