Perhaps the approach of forgetfulness in visual terms is not always appropriate, since we hardly see in our minds even the recent things or persons.
It happens that the old memories which are about to be lost to be fainter than some recent memories, if the latter have grasped rather a superficial feature of the person or thing used as an object of memory.
The process of forgetfulness borrows much from the sense of touch. Thus, we forget when we cannot grasp the object. The grasp is fettered because the mind goes through the object to be remembered.
In the case of a person, we start to forget her when the mind notices a lot of facts about her. The meanings of these facts begin to have a life by themselves and leave aside the person who caused them, so that she comes to be perceived only like a gate through which our thoughts may go further.
According to this explanation, the phenomenon of forgetting the close persons once they disappeared is not surprising. The usual experience of sexual contact happens under the same pattern. We go through the closest persons or use them for going through us in order to reach a personal pleasure which is something different from them.