While visiting the planet in 2151, Captain Jonathan Archer and the crew of the Enterprise came upon the Eska hunters. One of the Wraiths appeared to Captain Jonathan Archer as a woman he recognized; she had read his mind and taken the form, Archer later realized, of the "glimmering girl" in Yeats's "Song of Wandering Aengus" as he had imagined her when he was a child. She explained that her people were shapeshifters, and the Eskans hunted them using sensors to track them by identifying the Wraiths' chemical signature they give off when they are afraid. Dr. Phlox made a compound that masked their chemical signature concealing them from the hunters and hid them from the Eska's sensors.
Once her people had protected themselves with Phlox's compound, Archer's mysterious friend thanked him and let him see her true serpentine form, commenting to him as she departed that she had taken that form to remind him to never stop searching for what seemed to be unattainable.
The shapeshifting powers of a Wraith are very formidable. For example, if they become a rock then that is what they become and that is what sensors will pick up. This would explain why Sub-Commander T'pol was unable to detect any lifesigns on the planet. (ENT: "Rogue Planet")