You can never take the guard out of a guard dog. no matter how submissive that dog is. If there is a threat all members of the pack defend. Also, since dogs can read the moods of other beings the dog will know if someone has bad intentions.
A dog will not ignore that if they think you are in danger. The other point is, if you know for a fact that your dog is perfectly balanced and he starts acting out of the ordinary, that is, when you will know for sure that he senses something that is not quite right. As submissive as my Boxer Bruno is, he still will bark at the door if he sees someone coming down the driveway.
I could possibly teach him not to bark, but I can never take the guard out of him because it is an instinct.