I don't have much of a medical background but it seems to me that even if this node was not in the area of the surgery but maybe below it, if the PET was done shortly after the neck dissection, that node would be affected by having 30 lymph nodes above it being removed--the lymph channels leading to it would all be changed and then there might be more activity there just from the effect to the lymph pathways of having surgery. My guess is this is why the doctor didn't tell you about this. And, as you said, since it is in the field of radiation anyway, if on the off-chance it was due to cancer, the radiation should zap it.

Nelie