My Chihuahuas are great watch dogs...their hearing is out of this world. They hear things before my Doberman does sometimes. And they bark. And they bark. And they bark. LOL.
My Doberman started showing his protective instincts at about 4 -5 months of age...that low, throaty growl and alerting to things that were not normal occurances. At 8 months, an incident occured in the middle of the night where someone came into the house univited. We both jumped out of bed and he ran in front of me and stood, growling/barking viciously at the person, but didn't lunge or bite.
There was another weird thing where a strange man came into the house with my niece to let the dog out of his crate for me. This was before he was trustworthy in the house loose.....as an older pup (he took till he was like 17 months till I coud leave him loose in the house while I was shopping or something). I was not home. He knows and loves my niece, but growled quite viciously at this man with her. It worried her and she told him to get outside NOW and go in his car. It's strange how with my niece, he wasn't OK with this. If I had been there and let the guy in, he would have been fine.
Anyone I'm friendly to, he's fine with and will be quite friendly after a few minutes. He watches the person, then my face and back to the person again. A lot of his assessment is taken from my mood toward the person.
He has the reputation of a Doberman...will stand his ground and not back down, but has an uncanny ability to discern between a friendly stranger and one who is up to no good. Absolutely fascinating how he can tell. Delivery people can come up to the door or even sales people. He'll bark when they're first coming up the driveway and then just watch. He's suspicious of strangers at first, but doesn't fly off the handle. A lot of that also comes from extensive socialization....so very important. How else can they know what's normal and what isn't? I think a lot of the dogs who ruin their reputation are either poorly bred or not socialized enough. They are unsure of things and then more apt to bite. They have to be confident, fearless and educated about things. Then they do their job the right way.
Like Renee said, anything that is out of the normal, these protection dogs really notice. My Doberman isn't one to lie down, stay put and let the world go by. If there's any unusual sound outside, he's up in a flash, checking it out. This could drive some people nuts, this restlessness, but it makes me feel more secure. And I've gotten use to it.
He's still young...a little less than 2 years old. It will be interesting to see how he matures....at about 3 or so...that is when my trainer I had said that he'd turn into a Doberman. LOL. She had a Dobe at one time. She said to be careful...as he's learning about things, to keep him from going balistic at the window. Her's knocked down her mini blinds and almost broke through the window to get to someone. She said that I have to let him know that barking is fine, but not to go so nuts that he breaks a window. A-h-h-h.