Unknown volumes are currently not supported by the Usage plugin included in version 3.4.0. As a workaround we recommend excluding the respective volumes via the X switch after having verified their unknown state using -explore.
As of the next release (3.4.1) the plugin will skip unknown volumes (and outputs the number of skipped volumes). In verbose mode (-v) the reason for the exclusion can be inspected.
$ perl check_netapp_pro.pl Usage -H 18.104.22.168 -o volume -v NETAPP_PRO USAGE CRITICAL - 47 volumes checked, 9 critical and 21 warning 4 skipped. (-v shows reason) (...) Usage 203 Volume 'FCP_SAP04SE' skipped because state is offline Usage 203 Volume 'FCP_SAP04SF' skipped because state is offline Usage 203 Volume 'c_kanu_1' skipped because state is unknown Usage 203 Volume 'c_kanu_2' skipped because state is unknown