Full Text Search indexes all documents in a database.
Because this includes all mail messages as well as calendar and to do items, folks who are restricted to reading or managing calendar and to do items do not have access to this index.
You can make use of one of the other two remaining search methods .
This is a flaw in my opinion and one that I know that the developers have been asked to rectify. I have not found any information about whether it has been addressed for the upcoming 8.5 release.