USPS- I got a parcel locker key in my mail box.

This week I got a package from amazon and package tracker says USPS is delivering via priority mails. At the same time I find a key in my mail box.  Now what that can be? that is my parcel locker key. Now where is my parcel locker? There are no other mail box clusters in my apartment area. I have seen in some other apartments there are array of big mail boxes in the lobby or separate parking area. 

Now if I had observe my mail box cluster I would have find there is two medium size lager boxes underneath the cluster and there are two key holes. I was able to open one of these boxes using locker key. Viola.. ! there was my package. Now if you try to get the key out you can't get it out. I think postal clerk uses a special key to release it so the cycle continue for the next person comes in.