Answers:
One possible answer for this question would be: a secret room.
Step-by-step explanation:
It the setting of the story is a house, the image that would be best to symbolize the theme of secrets would be a secret room.
This room can be unkown for all of the memebers of the house or can be kept by one of them.
The secrecy of the room would be surrounded by hidden keys and mysterious hallways.