If a rushed manager says the door is fine but it’s left unlocked, who’s at fault?

I've been working at mcdonalds for a few months. I work closing. We go out the back door sometimes when we leave for the night, and we have to make sure the alarm is aarmed. Tonight I was closing with the gm and another crew member. The gm really wanted to go home so he was rushing us. When we were leaving we all walked outside. I asked "want me to just slam the door behind me?" He said yes. When the door is locked and the alarm is armed, it will make a loud peeping sound 15 seconds after it's closed then it will stop. We didn't hear it go off. The gm said if the door is closed then it's fine. I notices the door wasn't flush with the siding like I think its supposed to be, but it's seemed like it was closed. He said it's fine and let's just get out of here. I don't know if he knew it wasn't flush or not. I'm afraid the morning crew will realize it wasn't locked, or even worse someone will break in.

If a rushed manager says the door is fine but it’s left unlocked, who’s at fault?
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