Dogs act very differently with strangers that the dog sees accepted by the owner then strangers who are unknown. Have you never been growled at by a dog that is tense and bristling? Or had one lunge to bite you?

Yep. I have no doubts than in such a situation my golden retriever would dive behind me...I love that little derpy bundle of love. Not that anyone expects a Golden to be a guard dog.

There's a YouTube video that I can't find where three greyhounds are in a house, when someone kicks the door in. The greyhounds panic and hide behind the furniture while the guy helps himself to the contents.

Definitely not good guard dogs.

Found the video: https://www.youtube.com/watch?v=e7mspVXGoik

There are two greyhounds, not three.

If I was downvoted for saying that greyhounds aren't good guard dogs, then whoever did that should probably actually meet some greyhounds. The muzzles they wear aren't to stop them biting people, they're to stop them nipping at each other during races, and to protect small dogs and other animals when they're not racing.

They're sight hounds - silent watchers with an unbelievably friendly, trusting, and loving nature, and once they've tired of your attention they will wander away to sleep off the excitement.

Ditto whippets.