OpenInsight supports updating indexes directly on the server by running the index update process through the Windows Task Scheduler. If you use that process it has probably silently failed at some point leaving you without indicators until customers call with problems. The article walks through setting up notifications for failed indexes.

This blog posting is a tutorial that walks through setting up a Nagios passive check that can be securely updated with WGET or CURL.

If you're using Ubuntu 12 LTS "Precise Pangolin" with the Nagios 3.4.1 package from the Ubunutu repository and you're having trouble with passive checks and the freshness_threshold directive you may want to consider upgrading Nagios.