Looking at the issue again--it does not show the body in the coffin after Eradicator absorbs its energy/DNA matrix--but it doesn't show the coffin empty, either. So it's possible that the Eradicator's body isn't the same as the one in the coffin, just a near-duplicate. Then someone or thing stole the corpse after Eradicator left, but before anyone checked to see if the tomb had been tampered with.