You've probably seen an arrangement like this at certain shopping centers (this one's near a grocery store), or you might even have or have had one where you live. What is it? And why is that key in the lower-right just hanging there?
Each of these things is a collection of mailboxes called a cluster box unit. There are three CBUs in this photo, and they have a couple of neat features that I want to highlight.
Each of the numbered compartments is one tenant's mailbox. They receive all their regular mail—bills, statements, circulars, you know—in their mailbox, to which they have a permanent key for the duration of their tenancy. Mail is delivered not directly to their door, but in their numbered box in their CBU. The local Post Office knows which box goes to which tenant's address.
The drop slot halfway up the right half of each CBU is for outgoing mail. If a tenant (or, really, anyone if they feel bold enough) wants to mail out a vaguely letter-sized mailpiece, as long as it will fit through that slot, they can slip it in there rather than have to find a Collection Box (blue freestanding mailbox) or go to a Post Office.
The real interesting bit, though, is those two large compartments at the bottom.
Those are called parcel lockers. As the name implies, they're for receiving packages rather than letters. They're a shared resource; any tenant may receive a package in any parcel locker. And the way they're allocated and secured is clever as hell.
You'll notice that one of the parcel lockers has a key hanging in the lock. What's up with that? And how do I know that it's empty?
If you look closer, you'll see that each parcel locker has two keyholes. One is for the postal carrier, and the other for the tenant/recipient. The recipient keyhole is the one the key is hanging in.
If you were to walk up to that CBU on the right and try to pull the key out, you'll find that you can't. If you manage it, congratulations on becoming the new King of England; you'll need the diplomatic immunity when the USPS comes for you.
What about the other parcel lockers—where are their keys? Those keys are sitting in some of the tenants' mailboxes, likely on top of that day's mail.
See, the way a parcel locker works is, when the postal carrier shows up with your package, they open the whole CBU at once (the whole front just swings open), then put the package in, then use their key to allow them to pull out the recipient key for that locker so they can put it into the recipient's mailbox.
[I've edited that last paragraph; I had previously asserted that the postal carrier used their key to unlock and open the parcel locker specifically. I know postal carriers open the whole CBU at once, so unlocking specific boxes doesn't make sense—upon considering that, I think the second keyhole is just for freeing the recipient key.]
When the recipient arrives to collect their mail, they'll find that on top of their bills, statements, circulars, etc. is a key. The key probably has a tag that indicates which parcel locker it belongs to (you can see such a tag hanging from the key in the photo).
The recipient finds the parcel locker indicated by the tag, unlocks it with the recipient key, and retrieves their package. They can close the door, but—just like you—they can't pull the key out again without the postal carrier's key. It's meant to stay there, until the next time the postal carrier has a package for someone.
So the parcel locker in the photo that has a key hanging from it is empty. I know that because if it had a package in it, the key would be in someone's mailbox, waiting for them to collect the key and in turn their package. That's the state of the other lockers: each of those has a package in it, and its key is waiting in some tenant's mailbox for retrieval.

