Villaraigosa Defaced at LAX in Security Area

With one TSA Officer and one other security guard stationed on both sides of this "Welcome to Los Angeles" sign in LAX's Terminal 7 right before passengers enter the baggage claim area, how did this small tooth-gapping graffiti happen?

In fact, we noticed this first in August when traveling without a camera handy. Back then, only one tooth was filled in. Yesterday, more than three months later: two teeth.