FWIW I haven't found a way to get access to unshared files until after you remove access from someone. The Admin SDK only provides usage reports. The Drive SDK allows you to get files et cetera, but only for the current user (!) -- so that doesn't help for anything you don't have access to, which I'm guessing from the phrasing of your question you already knew :)

I'm guessing this is a feature? As in Google wants it to be easy to just create local sheets/docs/... without them being shared with your org by default and would rather worry about giving admins better features to manage and audit sharing than give admins access to every document under the sun :)