But as Glaurung was passing over Narog, Mablung drew aside and lay under a rock, and remained; for it seemed to him that he had an errand yet to do. He knew now indeed that Glaurung abode in Nargothrond, but he was bidden also to learn the truth concerning Húrin’s son, if he might; and in the stoutness of his heart, therefore, he purposed to cross the river, as soon as Glaurung was gone, and search the halls of Felagund. For he thought that all had been done that could be for the keeping of Morwen and Niënor: the coming of Glaurung would be marked, and even now the riders should be speeding towards Doriath.
  Glaurung therefore passed Mablung by, a vast shape in the mist; and he went swiftly, for he was a mighty Worm, and yet lithe. Then Mablung behind him forded Narog in great peril; but the watchers upon Amon Ethir beheld the issuing of the Dragon, and were dismayed.

“The Journey of Morwen and Niënor to Nargothrond” - The Children of Húrin - J.R.R. Tolkien