How do I retrieve the key from the key cabinet?
Well, first you need to unlock it.
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Or, lacking a key, force it open.
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Where have you last seen some tools?
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Get a crowbar from the toolbox in the lift. Use it on the key cabinet.
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Now all you have to do is slide the key you need to the unlocked position.
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Which one was Vane Sojourn's room again?
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You need to slide key 15 (the bottom one in the second column) to the unlocked position. Make sure to keep track of where your key is at all times.
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Armed with the key, you can now get a closeup to the door to cabin 3015 and unlock it.