From my experience they don't act like they are guarding unless they need to. Kryten will be 7 in January and I can count less than 10 times that he has gone into full guard mode. Most of the times he just watches but is more than willing to be pet by everyone he sees. Half of the time he has gone into guard mode have been a non-resident opening the front door when I've been home. The same person can enter easily if I'm not home. Other times are when he doesn't like how someone is acting nearby. Once he went into full guard mode for no apparent reason. Just because they don't seem like they are guarding doesn't mean they aren't, it just means they don't see anything to be concerned about.