Board Thread:Questions and Answers Forum/@comment-24171394-20150210125832/@comment-1158256-20150210231858

All enemy types can vary their level. If it was a Thrall, it was the Hand of Crota. Since you don't get the missions from Eris until you're level 20, that thrall will be level 20 at a minimum. If you are too far below the enemy in level (4 or more, I believe), the indicator will show as ?? instead of a level number.