User blog comment:IamJakuhoRaikoben/Chapter 406: The Girl Within the Crystal, Review./@comment-117.205.5.87-20141028094333/@comment-7418318-20141028095417

It could be a false lead so others can easier swollow the lie that she died, rather than being frozen.

It could be that she really is dead and they only froze her dead body. But they wanted a better place to mourn than below the guild, so they made the grave on Tenrou. Remember that Lisanna had a grave (or even two in the anime) even though there was no body to bury.

Or the grave could be part of the spell that keeps her in suspended animation.