I've gotta tell you a story (probably told it before) for those with super friendly retrievers and such. My Lab, Bonnie (rip) was like that....loved EVERYONE. When a stranger would come up the long driveway we had, she might give a couple woofs but her tail would sway back and forth and she'd go greet the person, expecting nothing but a nice social visit....very, very friendly.
Well...one night, my hubby and I went out and left the two kids at home...they were teen agers, one a young teen. And when we got home, they had been on the roof for hours in October, freezing. They reported that they were upstairs in their rooms and Bonnie heard a sound and ran down the stairs to the front door. And she was unglued! Snarling, gnashing teeth, barking viciously. The kids were so scared they went out my daughter's window and sat on the roof. They heard in the dark footsteps in the gravel retreating. When we came home and looked around the door, the jam was damaged...cracked and split like someone was trying to force it.
The same thing happened with our GSD when the kids were very young. I was home though, in the shower. And the door jam wasn't wrecked. But our GSD went ballistic at the door. And he typically was pretty accepting and friendly to most strangers.
So, you never know. I think dogs, when well socialized as puppies can often detect something we don't. I think they're able to discern better than we, a friendly stranger from someone up to no good.