I have to be honest, and say you are the only person I have seen this happen to. It would lead me to believe its very likely that member wasnt actually in the group you thought they were. If its not a problem now, we can leave it there, and if you see anything like this happen again please let us know. Alternatively, my first call would be for me to get a backup of the database from prior to that point in time, in order to check the groups. As you mentioned yourself, there, its not possible to move a user from a group without it actually being reported in the members page