*laughs* I'm a secretary, and my pens disappeared ALL THE TIME. When I worked in a smaller office, I had a solution.
I labeled all my pens in masking tape with the word "MINE" on it. At the end of the day, I'd go around to people's desks and take them back, saying "Mine, mine...mine..." as I took them.
Soon they started getting their own pens from the supply closet :)