This was intriguing, someone excommunicated from the LDS church who wasn't a member! As a church member myself, who has some responsibility over membership records in our Ward, I can tell you that there is no way a person can get into LDS church membership records without either (1) being
baptized and
confirmed, or (2) being the child of someone who was baptized and confirmed.
This being said, it is certainly possible that someone who is baptized at age 8 (the minimum age) and subsequently never attends church (because their family never does afterwards), might forget ever having been associated with the church. I was baptized at age 15 (I'm a convert), so I definitely remember that event, but I cannot tell you anything at all for certain about events that occurred when I was 8, or around that age, unless they were fairly memorable. Someone whose family is basically inactive in church, though believing enough to want to have their children baptized, might grow up knowing virtually nothing about their church membership. But since church records are maintained scrupulously, someone might forget the Church, but the Church never forgets them, and in the programs set up for the purpose of ministering to members' needs they may be visited from time to time to check on their welfare. In my experience, most members know that they are members, even those that want to keep their association with the Church very tenuous, but there have been a very few who seemed to be completely clueless about it (although I could have told them the dates they were baptized and confirmed if they had asked).
In the excommunication written of here, I am saddened by the behavior of the home teachers, and of the authorities in this case. In a perfect world, it never would have happened, but I've noticed we don't live in such a place, mores the pity. We have a few members of our Ward who get very obnoxious if someone from the church contacts them, even if it only happens once a year. The weird thing is, even though they can ask to have their names removed from church records -- without prejudice, such as through excommunication -- they blow that off, too, if it is offered. It's like they're saying, "We want nothing to do with you, but we refuse to leave!"