My Austrailian Shepherd will do all she can to let me know there's a stranger afoot. But if he gets in the door before I wake up, there's a real chance she'd sell me down the river for the price of a cookie!
Not really sure about that, but I've never observed any real, agressive behavior from her towards a human. But she has demonstrated a willingness to loudly announce the presence of strangers, hence her name, "Gracie Barksalot."
The cat tells me he's not in that line of work, to begin with.