That's unfortunate, @Sophia. Are you absolutely sure a guest has taken the passport?
I may be wrong, but I doubt that AirBnB would cover the cost of a replacement, as it's the host's responsibility to keep valuable items in a safe place to which guests would not have access.
You could contact AirBnB and report the alleged theft. Have you contacted the guest(s) about the matter? If you're absolutely certain the passport has been taken, you should probably alert the US embassy in any case so that the passport can be blocked if anyone tries to use it.