Flashback to the roaring 1920's, the prohibition fueled gangster era in America. You won't need to be a wayward millionaire bootlegger to find this cache. But keeping in spirit of the era, you'll want to stay on the 'down low'. One can never tell who is watching. Friend, they are watching. Always watching, you can feel their eyes upon you. Speak-easy's were a safe place of refuge. Entrances to them were often behind hidden doors or in underground passages. GZ is near something that while visible on the surface mostly operates underground. What's in that box is almost as mysterious as a speak-easy's location back then. Again, friend, the cache is very near there not in there. Whatever you do, don't go in there.
Don't worry, there aren't any secret knocks needed to find the cache.