A lot of members who have left for various reasons still check in as themselves and just don't say anything. When you look at the profile of a certain member, you can see when they were last active. I suppose there may be some who just log out and look in as a guest, but since you can't see everything when looking in as a guest, I would think they would occasionally go ahead and log in and have a look around. I think it would be hard to resist.
I don't think there's any way to tell the difference between a logged out user and an actual unregistered guest. Why do you ask?