You're doing it wrong, prefab is one thing, instantiated prefab is another.
You need to save the object as variable on runtime. So, use get owner on the object you want to find and save it as gameobject variable, then use find object to find it. Bear in mind that you will have to save it as a global variable if you want to find it that way which is a no-no, or search by string which is inefficient.
I recommend saving objects and other data to arraylist or hashtables, then save the arraylist or hashtable proxy as a global variable so you can extract all objects and data from there.