I don't know if you follow boxing, but boxers have an term called the "one-two", where you hit your opponent in one place, and then the opponent expecting a hit in another place, gets the same hit in the same spot---again.
Buy your g-f some cheap knives--good, but inexpensive. Go to a cutlery supplier that supplies chain restaurants with rental knives. Buy them there, they are butt-ugly but work well. Because, you see, if you replace knives that got stolen with nice knives--again, they get stolen (probably by the same person)-- again. The ol' one-two.
Keep the nice knives at home, at work with use the butt-ugly ones that do the job well.
Get her a tool box for christmas. Not a knife roll, but a tool box, one that is too big to steal without getting caught, and one that she can put a padlock on when she goes for lunch or a bathroom break. Then get her to plaster the toolbox with stickers so everyone knows it's hers.
Hope this helps...