With active checks, regular service windows can easily be handled using time periods and check intervals. How do you handle this for passive checks?
You can set an interval in which the passive check changes state if the server has not received any passive checks in that time interval. This is known as freshness in Nagios 3.0 terminology.
When the freshness interval expires w/o a passive check, naigos runs the "active" check for the service. You can set this to something like
check_command check_dummy_fail!"No passive service notifications"
and you will be alerted just as if the passive check had failed. The tricky part is figuring out exactly what this interval should be.