you should be able to find this in the analytics DB. More info here: https://community.jivesoftware.com/docs/DOC-4778
The searches are stored in the jivedw_activity_fact table (activity_types 170,180,190) and the search strings are in the metadata column. However, I do not think these events are rolled up into the aggregate tables.
For reference, the 3 search events captured are:
170 - SEARCH_CONTENT
180 - SEARCH_USERS
190 - SPOTLIGHT_SEARCH
Thanks Josh, it was helpful.