Well, in traditional small town cultures, I think it's normal for neighbors and friends to freely come and visit, since they trust each other, and everybody knows each other. A childhood friend would normally simply walk into another friend's house when they're something like 5-12 years old. So if they're still close, even at the age of 16, I think it would still be normal. She already said that she rang the doorbell several times. So it's not odd that she would come in if the front door was unlocked, in order to check up on a childhood friend or neighbor if you're worried about them.