DM: Try using your large selection of keys.

You do have a lot of keys. A ton of keys, in fact! Mr. M just keeps giving you more keys to more places, and you don't have a good way to keep track of them, so you just try all of them. Not that it really matters, because none of them worked - makes sense, why would you have a key to the office? Unless... unless one of those four broken keys you find while trying them all are a match.