Parents, Padres but especially the mothers/madres/mamas
Be on the look out. As most ninjas, they attack quietly and quickly. They are called ninja "duendes" (elves).
Here is my story (Law and Order theme): I had just cleaned up the room full of toys. Everything was in its proper box or bin. It was one hour before bedtime. My husband was using the computer when the ninjas attacked. The ninja duendes quietly came into the room and the damage was done! Chuy didn't hear them, didn't see them (was it because he was probably on Craig's List? quien sabe) but they left their mark.
Should you happen to catch one and ask them why they attacked your home. He/She will simply look at you and respond "That's why."
Be aware that they are disguised as your children. As a good mommy, I am getting the word out.