In the episode with Fry and the 7-leaf clover, a flashback scene shows how Fry hid the 7-leaf clover in a safe with his Breakfast Club Soundtrack LP. In the flashback, Fry shuts the safe and walks away.
One thing: After closing a safe like that, you must clear the tumbler! In other words, you have to spin the thing you turn with all the numbers on it a few times. Otherwise, the safe isn't really locked.