I've heard of some straight up horrible things happen to dogs when their owners were away. One of my friends came home from work a few years ago and found his dog with a screwdriver through its head. I've heard of oven cleaner being used. Truly fucked up.
I'm a decent sized guy that has been attacked a couple of times by dogs as well (a pit and a doberman), and while I have some good scars, I wasn't ever too worried about handling myself against them. I am scared of rottweilers, but I can easily out run one of those fat fucks. Multiple dogs are really needed to scare off an experienced thief. Even then, don't grow too attached to a guard dog. They don't have great chances if someone wants what you have.