When they stay in the wrong house...

I really adore the movie, but I've always been curious about the scene in the beginning where they first arrive at the beach house and are woken up late at night after the owners come home and are surprised to find this family of strangers sleeping there.

Now, the landlord told them the keys would be in the mailbox, which they were ... an interesting coincidence since they're not at the right house. If the house wasn't for rent, then why were the keys in the mailbox and why are the owners then surprised to find people in their house? They didn't seem to be expecting any renters.

Do these people just have a habit of leaving their house keys out in the mailbox so anyone can come on in? If they're a set of emergency keys, that's a crackerjack place to keep them.

I understand it's all for the sake of setting up a funny scene and I'm willing to overlook innocent continuity errors in a comedy, but this bit just makes no sense.