This is an interesting one, Google is always testing features and it seems I missed this back in late January. But Google is testing one where the auto-complete feature is showing you trending searches, as opposed to just ordinary most common searches, as you type. The difference between trending and normal auto-complete is basically the difference of showing search suggestions based on years of query history vs the past 48 hours of search query history (in the most simplistic way).
In any event, here is a screen shot showing the trending symbol in the auto-complete. Notice the arrow on the left of the term:
That arrow on the left symbolizes the trending term.
What is interesting is that AJ from Google just posted in the Google Web Search Help thread asking users about this. He is treating this as a bug. He wrote:
Thanks for all of your reports here. We're currently investigating this, and would appreciate if you can share some details for us to troubleshoot.
When you're experiencing this, is your account logged in to the Google Search app? You can check this by accessing the Google app > Click three-bar menu. If your email address shows on the top left corner, you are logged in. If it says "Sign in", that means you are currently logged out. Thanks!
This is simply a limited test and it doesn't seem there is a way to opt out of it.
Forum discussion at Google Web Search Help.