Yeah, I am guilty of this! I know my boss would shoot me if he knew though!
I work in a shop and there are often times that its dead, so if I am working on my own I'll go out back and look through a few books and take notes for an assignment I am working on and when I see someone walking into the shop through the monitor, I'll just go back out.
But, if you are not working alone ... I guess that could be more tricky!