assumes that if it can't see, it can't be seen. Many people
Ever notice that people who are wearing headphones while talking don't speak at their normal volume? It's always at a volume to which they can hear themselves speak over the noise coming out of the headphones. Subconsiously, they believe that since they can't hear themselves speak, nobody else can hear them speak either. It's an interesting phenomenon, and I find that most people do it.
(This just happened to be a thought that popped into my head at random, and I was all set to node a brand new node - imagine my surprise when I found it already existed! Woah!)