Dec 2006 total thyroidectomy for papillary carcinoma & neck dissection bc in nodes...now us shows tissue in thyroid bed...what is it ?

Biopsy to know.: Recurrence of the thyroid cancer in the "thyroid bed" can occur. Given your disease seven years ago i would be suspicious of that. You can confirm or rule out that with: a biopsy, a metascan, follow-up thyroglobulin levels (if done regularly since) a pet/ct. It could certainly be something else/benign but i would have it thoroughly evaluated before coming to that conclusion.

Residual thyroid: The surgeon cannot remove every last remnant of thyroid. This is why RAI is often given after thyroidectomy, to mop up any residual thyroid tissue-normal or cancerous. The US may just be detecting residual thyroid that the surgeon did not get. See your Endocrinologist.

Left over thyroid: It is impossible to get all of the thyroid out. It is likely a bit was left off of the superior lobe, at berry's ligament, or a pyramidal lobe that was not appreciated at the time of surgery.