Mark Traphagen wrote on Google+ that Google has discontinued the Private Results option in the Search Tools section of the Google search results.
Truth is, I don't think they have. I just think it works sporadicly either by design or because of bugs.
Mark wrote:
Up until yesterday I could get a feature under the Search Tools link at the top of my Google logged-in search results that let me change "All results" to "Private." The Private filter gave first preference to Google+ content from my circles and to web pages I've previously visited.
We reported on the tool a coupe months ago as working for some and not for others.
The same difference here, some see it, some don't. Some see it working sometimes and some don't.
I guess there are a lot of variables to get it working here.
Forum discussion at Google+.