I've had total thyroidectomy & neck dissection & 2 rai treatments, ultrasound found tissue in thyroid bed, what does this mean?

Uncertain: The only way to tell what the tissue in the thyroid bed is would be to do a needle biopsy. There could be scar tissue. There could be recurrent tumor. If the thyroid tumor picked up rai then a nuclear scan could show if there is recurrent tumor corresponding to the ultrasound findings. Talk with your oncologist.

Left over tissue: What you describe could be scar, left over normal thyroid tissue, or thyroid cancer. Ultra sound guided fine needle biopsy, +/- molecular markers and a thyroglobulin wash out can give you the answer.