Where do all these mystery keys come from?
You toss old, obsolete keys every Key Purge.
You toss the laundry room key from two apartments ago.
You toss the key to a freight elevator located in a building that's been demolished.
You toss what might be a key to a gym locker, or it might be for the mailbox from the building where you worked six years ago, or else it unlocks a piece of luggage you're not sure you can even find right now.
You keep only the essential keys. But now you forgot why they were essential.
Quite the nagging worry, innit. Middle of the night, can't sleep.
You locked up something a number of years ago, but you don't know why you did, what it was or where the hell it is now.