Is it a bad sign, if reviews are completed very early? My manuscript was sent to the reviewers of an Elsevier journal around 10 days ago. Today, I logged into EES and saw that the status has changed to Required Reviews Completed. I couldn’t believe it. My impression is that my manuscript is likely to have been rejected by the reviewers. Am I right about this?
Required reviews completed means required reviews completed! Nothing more and nothing less. 

There are particular seasonal patterns for the journals, to be flooded with a lot of manuscripts at some times (lets say this time ~ May-June), while Sept-Oct are less hectic months. Likewise, referees, who are generally faculty members at universities, are less likely to review fast in the middle of the semester, and more likely in the period when they are free and not away for visits or conferences. It is possible that you sent your manuscript at an opportune moment. Another possibility is that the editor of the journal may himself have chosen to be a reviewer. Another possibility is, you used a method which was pretty bread and butter for the reviewer, so he didn't have to waste time trying to understand what you did. 

I can sense your surprise over the time period of just 10 days, but no need to panic here. (especially until the editor reaches out to you.) Nothing more can be said till then, reviewing over in 10 days may mean instant rejection, but it may also mean an acceptance if the reviewer was very well familiar and saw no issues. Nothing can be conclusively said until the referee reaches out to you. 

Hope that helps :)