RCModel nailed it early on.
I've never seen a tactical pen that doesn't scream TACTICAL PEN! to co-workers, LEO or security personnel. I received one as a gift -- S&W was kind enough to put their corporate logo on the clip, facing outwards toward all humanity. Great for advertising, lousy for secrecy.
There have been days I have had to leave my desk for a short walk a few blocks form the office; I have a metal Parker pen that sits in my hand or shirt pocket. It is about as invisible a weapon as possible.