As you guys would have known, you can view people who have logged in to your facebook account along with their location (city, country). But what you should know is that this feature does not seem to always work. In other words, its not foolproof. Now, I am not a 100% sure why but in my experience, it did not work when I was already logged in from one place and asked the second person to login. It just showed the information pertaining to my login and not the second persons.
I do hope that facebook would find this bug and fix it soon as internet security has become a pain these days.