There are locks that needs a key to open, but can be closed without using the key.
Imagine the following scenario: You are the only person in the home who owns the key to the shed. A week ago, you open the shed, store some stuff inside, check whether everything is in order, then exit the shed and finally lock it. Later the police come and find a dead body inside the shed. Since you are the only one having the key to the shed, the police suspect you as the murderer. However, the padlock to the shed can be locked without using the key. Show how the murderer can use that fact to enter the room without having the key.