Missing Parcel Key

I am missing one of the parcel keys with the blue tag on it. It has the number 6 on it. I am finding that many people after retrieving their package from the parcel boxes put the key back in their own mailbox.

The keys belong in the mail slots above or just to the top left of the box they got the package out of it. Could the key be returned as soon as possible. It has been missing since Thursday.

Thank you, 

Kevin (Postmaster)