/etc/nagios-plugins/ is in nagios-plugins-contrib 16.20151226.
This file is owned by root:root, with mode 0o755.
..
/etc/nagios-plugins/check-libs.conf
/etc/nagios-plugins/check_rbl.ini
/etc/nagios-plugins/config/
/etc/nagios-plugins/config/ajp.cfg
/etc/nagios-plugins/config/bgpstate.cfg
/etc/nagios-plugins/config/check_hp_bladechassis.cfg
/etc/nagios-plugins/config/check_uptime.cfg
/etc/nagios-plugins/config/cups.cfg
/etc/nagios-plugins/config/email_delivery.cfg
/etc/nagios-plugins/config/haproxy.cfg
/etc/nagios-plugins/config/httpd_status.cfg
/etc/nagios-plugins/config/ipmi_sensor.cfg
/etc/nagios-plugins/config/lm_sensors.cfg
/etc/nagios-plugins/config/memcached.cfg
/etc/nagios-plugins/config/mongodb.cfg
/etc/nagios-plugins/config/multipath.cfg
/etc/nagios-plugins/config/printer.cfg
/etc/nagios-plugins/config/rbl.cfg
/etc/nagios-plugins/config/redis.cfg
/etc/nagios-plugins/config/snmp_time.cfg
/etc/nagios-plugins/config/ssl_cert.cfg
/etc/nagios-plugins/config/webinject.cfg
/etc/nagios-plugins/config/whois.cfg
/etc/nagios-plugins/config/zone-rrsig.cfg
/etc/nagios-plugins/config/zone_auth.cfg
/etc/nagios-plugins/obsolete-packages-ignore
/etc/nagios-plugins/obsolete-packages-ignore.d/