Old scarring is non-specific. From a thyroid cancer standpoint, a non-contrasted chest CT could be performed to correlate the findings on CXR - the comment of "looks like scar" is not precise enough. But this needs to be taken into context of your thyroid cancer risk -- ie, thyroglobulin levels and neck ultrasound primarily. PET and I-123 are sometimes helpful as well.
Why an x-ray for a followup? That wouldn't show anything until it got quite large.
Usually they use I-131 or I-123 for a thyroid cancer followup (unless you had anaplastic or medullary) or a PET scan.
Where was your scaring?
Probably some calcifications. I had those show up too, and they said they were likely leftovers from a bad bronchial infection or pneumonia.