As reported by Yslow

(no expires) moodle/theme/standard/styles.php
(no expires) moodle/theme/standardwhite/styles.php
(no expires) moodle/theme/standardlogo/styles.php
(no expires) moodle/lib/overlib/overlib.js
(no expires) moodle/lib/overlib/overlib_cssstyle.js
(no expires) moodle/lib/cookies.js
(no expires) moodle/lib/ufo.js
(no expires) moodle/lib/dropdown.js
(no expires) moodle/calendar/overlib.cfg.php
(no expires) moodle/theme/standardlogo/logo.gif
(no expires) moodle/mod/glossary/icon.gif
(no expires) moodle/mod/resource/icon.gif
(no expires) moodle/pix/i/user.gif
(no expires) moodle/pix/i/course.gif
(no expires) moodle/pix/i/item.gif
(no expires) moodle/pix/i/closed.gif
(no expires) moodle/pix/moodlelogo.gif
(no expires) moodle/theme/standardwhite/gradient.jpg

Above files does not contain a header. Somehow parts of Moodle is not respecting http://docs.moodle.org/en/Apache#Performance

mod_expire is working for some parts of Moodle and for all static html pages so this issue has to be present in Moodle.