Due to this bug:
I'd like to have dash forget what I searched previously so future searches are not problematic. This would make Unity more consistent with retrieving the items I am telling it to. Is there any way to do this? I tried to toggle off the "history" in system settings >> privacy, but it did nothing...
You were looking in the right place. Go to 'Security & Privacy' and go to the 'Files & Applications' tab. Toggle off 'Record file and application usage'. Also be sure to click 'Clear Usage Data' to clear anything that is already cached.