Normally this is prevented in previous versions of OSX Spotlight, which underwent a rework for Yosemite, indexes desktop files and makes them searchable via keywords. By default, the latest iteration of Spotlight sends those search terms to Apple, in conjunction with Microsoft's Bing search engine, which Apple says is used to improve Spotlight's accuracy. If "Location Services" are turned on in Yosemite, a computer's approximate location is also sent to Apple whenever Spotlight is used.
Disable "Spotlight Suggestions," "Bookmarks & History" and "Bing Web Searches" under the "Search Results" tab in Spotlight's preferences to prevent this. An XCode macro (Python Script) is included in the article to do this.