Strange goings on indeed! Those members who have chosen to be "invisible" still show as one of the first three categories whenever they are logged in. They are merely not named. However, if it was someone who had logged in, they would not appear as a guest. It therefore has to be either one of the Staff or a Member.
What is really odd is that I also saw recently "1 Invisible User" when I was the only person whose name appeared on the Info Centre. Yet even the names of those who are "invisible" can be seen by Admin! Spooky!