Answer:
The best answer should be a keyhole.
Step-by-step explanation:
Keyholes represent the entrance for every house, for every room of the house, and even for every closet inside the house, besides representing vault's entries and more. If there is no keyhole for all these examples, then there is no entrance for each of them, what consequently makes everything inside of them a huge secret for everyone who wants to enter in but is not able to.