People milling about are a common sight to those passing through airports. Whenever I'm sitting at a gate waiting for my flight to board, I see lots of people walking about who look like they're not sure why they're there.
People who are at airports shouldn't not know why they're there. Airports and everything in them have clearly defined functions. So clear in fact that I imagine people slip through the cracks every once in a while -- find their way into some dark corner in the airport which (gasp!) serves no purpose.
These people get lost in a sort of alternate state of mind, where there is nothing but the airport. You can see them, but they can't see you. And there these airport denizens spend the remainder of their days, buying food with their credit cards since they never have to make their payments anymore anyway, bindly staggering to whatever gate the PA tells them to, having long forgotten what their destination actually was, and sleeping in the first-class lounges with their luggage as pillows.
Some airports don't have them at all, some are full of them.