try looking in historical_event_types on the database. I has a look and it doesn't appear that there is a search activity recorded within the historical_events. If it were anywhere, this is where it would be.
select * from historical_event_types where upper(name) like upper('%search%')
We don't record search history in the database at all.
The searchserver logs do capture every call made, so each call from the search box is there if you need to see a history. The calls are a bit cryptic but can be parsed out.