Thyroid u/s showed nodule in isthmus: 9x3x7mm, slightly hypoechoic, markedly vascular. At biopsy, neither dr could locate it. Should I be worried?

Try not to. Get a followup ultrasound in 6 months. Thyroid nodules are very common but these nodules are rarely cancerous. If it gets bigger they may need to try again for needle cytology.
No. Bad disease is not hard to find. At the current measurement it is still very small. I would do another ultrasound in 1 year. If it's still small or can't be found, do nothing then. No worries.