SanAndreasUnity/Assets/Scripts/Behaviours/OutOfRangeDestroyer.cs

using UnityEngine;

public class OutOfRangeDestroyer : MonoBehaviour
{
    public float timeUntilDestroyed = 5;
    public float range = 250;
    public Transform targetObject = null;

    private float timeSinceOutOfRange = 0;

    private void Start()
    {
        if (targetObject == null)
        {
            if (Camera.main != null)
                targetObject = Camera.main.transform;
        }
    }

    private void Update()
    {
        
        if (targetObject == null)
        {
            if (Camera.main != null)
                targetObject = Camera.main.transform;
        }

        if (targetObject != null)
        {
            // only increase time if target object exists
            timeSinceOutOfRange += Time.deltaTime;

            float distanceSq = (transform.position - targetObject.position).sqrMagnitude;
            if (distanceSq <= range * range)
            {
                timeSinceOutOfRange = 0;
            }
        }

        if (timeSinceOutOfRange >= timeUntilDestroyed)
        {
            Destroy(gameObject);
        }
    }
}