In my mind I see the NOT lamp as a running joke amongst the crew. They challenge each other to see how many scenes they can sneak the lamp into. Think I'm crazy?
Series 1, episode 1 - IT office
Series 1, episode 1 - Big boss' office
Series 1, episode 4 - IT office (we can still see you)
Series 1, episode 4 (#1)
Series 1, episode 4 (#2)
Series 2, episode 4 - Jane's ORGELriffic apartment
Series 3, episode 4 - Lab* (TERTIAL work lamp on the right)
So with the exception of series 1, episode 4 it honestly looks like they keep moving the one NOT lamp from scene to scene. It is a $9.99 lamp and they are treating it like the $100 pair of shoes you wear every chance you get in order to get your money's worth. It is a nice, simple lamp though.
*The lab gets a post all its own on Friday.