Check_unmonitored_hosts is a project mainly written in Perl, it's free.
Nagios plugin to check unmonitored hosts
Scans the given subnet for hosts and checks to make sure that all hosts discovered appear in the opsview database.
These requirements should probably exist by default on most opsview installs
The best way to set this up is to add an Unmonitored Host check to your opsview server with the arguments:
-w 0 -c 5 %SUBNET%
Then, for each opsview host, apply the check and under the attributes for the host, add a SUBNET attribute for each subnet you want checked