Has anyone seen the show on Discovery called "It takes a thief?" It's about 2 ex cons that help people be safe. Every time he was in a house with a big dog, he still robbed the place. If i remember right he was actually more afraid of the little ones since they tend to cause more noise and draw more attention to the house. The big ones mostly took a bone or after being locked in a room, laid down and went to sleep. The little ones followed him around the house barking really loud and biting his ankles. Of course I don't think these were trained dogs, so if you got a professionally trained guard dog you may be better.