Slightly tangential, but sometimes people I can only describe as "tourists" walk around in the publicly accessible areas, so it's not totally unprecedented.
For what it's worth, I tend to tell these people apart from the people with business in the building by noting where they're looking (tourists seem to be looking all over the place like they're taking it all in; people with business at Gates seem to be looking for numbers and other navigational indicators in line with tzs's comment).
And, as it happens, "Hey, are you lost?" can be a polite way to tell a tourist that they've wandered in to an area that they aren't really supposed to be.
For what it's worth, I tend to tell these people apart from the people with business in the building by noting where they're looking (tourists seem to be looking all over the place like they're taking it all in; people with business at Gates seem to be looking for numbers and other navigational indicators in line with tzs's comment).