What a bummer! Unless the guilty party was caught in the act or someone actually saw what happened, there's not much that you can do. You were told nothing like this has ever happened before, unlikely but possible. It's easy to understand why you're so mad, anyone would be especially when management won't follow up. Maybe until you can give your notice only bring what's absolutely necessary and keep your cash in a pocket. Sorry, wish there was more I could tell you.