Just go read it, I won't even try to construct a synopsis of everything in this article. Dawn probe spots dimmer companion close to Ceres' strange 'bright spot' | Daily Mail Online
Well duh. It's obvious what it is. It's a plaque placed on Ceres in the year 2032 by the lone surviving member of the failed Mars Colonization Effort who, in an attempt to return to Earth, encountered technical difficulties, traveled back in time and placed the warning on the only celestial body she could reach. How is that so hard to recognize?