the key

When you have to leave for a few days it's good to leave your key with someone you trust.
That way there's access to your apartment/house/room when something happens, or when you find out you need important papers.

Sometimes there's only one key.
Make a second when you have time, so you can hand out one and take one with you.

When it's not allowed to make a second one,
Give the key of the mailbox to the person of your choice and when you leave put the key in an envelope and put it in the mailbox.
Preferably tape it to one of the inner walls.

I wouldn't use this solution myself, unless there's no other way and the key isn't alone in there for longer than an hour.