Try a private message to a member named Utahmomma, she and her sisters have all had recurrance of thyroid cancer, and she can answer your questions.
What led you to get the ultrasound?
I would suspect the soft tissue density is more than likely some thyroid tissue. The nodule at this point in time is not very, if your post is correct. It is unlikely to do any damage nor be causing the body to even know it is there, hence the thyroglobulin test being undetectable. You could always get your levels checked in the near future to see if it is causing any thyroglobulin, but be aware, being on thyroid replacement medication makes it difficult to detect it, and you would need a different way of obtaining that information than through a standard blood test.
I suspect the doctor is doing a 'watchful wait' Whilst difficult for the patient to put up with the waiting, it is often the only course of treatment when there is no definitive cancer.
Cheers