With the release of Mass Effect: Legendary Edition in May, the sci-fi series has been garnering a lot of renewed attention, including from developers who worked on the original game. For example, a designer who worked on Mass Effect 3's 2012 release recently unveiled an Easter egg that he put in the game that had never been discovered.
Cinematic designer Richard Boisvert posted on Reddit in May about the secret and gave some guidance to players who were interested in seeing it for themselves in the remasters. It's a cool little homage to a Mars rover, probably Opportunity, near the beginning of Mass Effect 3's Mars mission. If you follow a specific route, you can find a rover that'll drive over to you and give you a little nod.
Image of rover Easter egg courtesy of Reddit user Tuskin38.
Boisvert himself believed that no one had ever even seen the rover, but that part isn't exactly true. As pointed out by Kotaku, the rover itself had been spotted last year--but no one had ever figured out how to trigger its action.
