There was once a group setting called "Exposed Db Host Check" which would periodically attempt to connect to your mongo servers to make sure they were not reachable via the public internet. This is now configured like any other alert called "Host is exposed to the public Internet." Any groups that had the old "Exposed Db Host Check" setting turned on had it automatically converted to an alert. Also, all newly created groups will get this alert from now on. When the alert is enabled, we will run the check twice a month, one the 1st and the 15th. Once we find an exposed host, we will continue to try to connect to it every 5 minutes until it is locked down and no longer accessible. Last, the email that we send out when an exposed host alert is triggered looks just like the other alert emails.
Note that this FAQ needs to be updated.