We have several dogs. The one that is the most laid back is mixed, mostly Spitz. If a stranger or someone he does not know is around, he will be in hiding. If they attempt to approach him he will just sling away. If I take him out on leash, he will hid behind me with I stop to talk to someone. He allows kids to "drag" him around and never gets angry.
Until, one day when I came through the door, unexpected. He was on wifey's lap and when I came in he was immediately jumped on the back of the couch ready to pounce until he realized who I was. We have now seen this behavior twice. While he is generally "laid back" and very shy, he does have a protection instinct. I have no question he would protect the house from an intruder. We were very surprised the first time we saw it.