Picture, if you will: A classroom full of students. The desks aren't really arranged, but pulled into random groupings of 2 and 3 and 4. Everyone in the room seems very intent on their notebooks or their laptop, talking quietly with those in their group. You look around the room for the teacher, and you can't seem to find her. Nobody's lecturing, and several people get up and walk around the room to talk to others about something before going back to work.
I actually had a student stop by and be unable to find me in the room (even after someone told her I was there) until I came over to her. Feeling truly invisible now!