Kenichi Suzuki, one of my favorite SEOs from Japan, posted a screen shot on Google+ of a search tool filter to filter results by private content shared with me.
Here is his screen shot:
I personally do not see this feature because I am a Google Apps user, but most Gmail users should be able to see it by conducting a search and selecting "search tools" and clicking on the "private" option in filters.
This type of filter will show you content that was shared with you on Google+ or Gmail if you are signed in to your Google Account.
It is similar to what they launched in 2013 where you typed "my photos" and so forth but this is now an explicit search filter.
I don't think this is new, but I never see it myself since it is not enabled for Google Apps users by default.
Forum discussion at Google+.