I have been living in the very same room in the same hall for about two years now.  Nothing about the room has changed.  It's the same bed, same door, same dull white paint job.  But earlier this week I was walking into my room and I saw this on the door.  So I asked my roommate, "Did you put this on my door?"  He told me it had been there ever since we moved in at the beginning of last year.  It was something I probably have seen every single day for the last two school years, and yet I had never noticed it.  I don't know what it is, but it's interesting.