In the third movie, Berkians were forced to move to the new home. What is Berk terrains like? Why does Grimmel the Grisly able to sneak in to Hiccup’s home so easily?
I asked myself the exact same question when I watched the movie. The Berkians probably don't lock their doors, because every time someone exits their house, we don't see any keys or something similar.
And my second thought is that they don't have any sentries near the forest where they saw the Lightfury for the first time, we can easily point that out because a trap was placed there too. So the Berkians should've probably made their "security system" a little better.