how can i make appear an image when target is not found or lost?

If you are looking for something that will show when something is not there anymore or is destroyed, I think this will be what you are looking for:

 public SpriteRenderer notFound;
 // This is for showing and hiding the not found message
 public GameObject target;
 //GameObject that you want to check if it is there
 
 void Update{
 if(target == null){
 notFound.enabled = true;
 } else
 {
 notFound.enabled = false;
 }
 }