Everything is relative. If you steal your sister's stuff and she doesn't mind, how is that relative to the serious details here?
When you have a group of girls you see often and return something later, I understand that scenario. But, I think it is important to try to figure out details according to the situation within the case. Not what you, me or your sister would normally do.
Same goes for the keys. The text read Christina could not find her House Key.
The majority of people want to sum up the situation according to their habit of keeping the house key connected to their car keys. The text didn't read that Christina couldn't find her keys. So, I would assume she had her car key (even if the wrong assumption) because the text only provided info about her house key.
It is frustrating to read blanks being filled in with what we usually do or think or by majority rule, most likely situation - etc.
Different people have all kinds of differing habits so how can we assume anything?