Did the 2.4.x upgrade complain about any mods/blocks missing during the upgrade? Those missing were the add-ons.
If one is talking about what one would see in the drop down (or now popup) activities and resources menu, those can be found in
moodle.back/mod/
folders/directory there are named fairly closely for the activity one sees in the Moodle UI.
Example of a stock 2.4.1 (no addons):
[root@sos moodle24]# ls mod
assign choice forum label quiz survey workshop
assignment data glossary lesson README.txt upgrade.txt
book feedback imscp lti resource url
chat folder index.html page scorm wiki
for blocks (add a block in Moodle UI), look in moodle.back/blocks/
Example of a stock 2.4.1 blocks directory:
[root@sos moodle24]# ls blocks
activity_modules completionstatus index.html online_users settings
admin_bookmarks course_list login participants site_main_menu
blog_menu course_overview mentees private_files social_activities
blog_recent course_summary messages quiz_results tag_flickr
blog_tags dock.js mnet_hosts recent_activity tags
calendar_month edit_form.php moodleblock.class.php rss_client tag_youtube
calendar_upcoming feedback myprofile search_forums upgrade.txt
comments glossary_random navigation section_links
community html news_items selfcompletion
But, there are other types of addons ... filters, assignment types off the top of this feeble brain.
There's a database way if you load up a previous backup of the DB into a DB named not to clash with current active DB. But one ends up comparing something stock there too!
'spirit of sharing', Ken