This patient HAS primary hyperparathyroidism, and there IS a missed adenoma in her neck. [Assuming this is the unflawed intact pth assay]
Tc Sestamibi is flawed, but helpful [although not in your case]. Orlo Clark found 71% of single adenomas with it (Arch Surg). MRI is even more flawed.
Is your colleague who explored the neck a regular endocrine surgeon? If he is not, my best advice at this stage is to refer the patient to someone who has done several hundred of these and has a special interest, as the problem of a missed adenoma is a special ball game in surgery, and requires special insights and expertise.
Such a surgeon would collect all existing evidence and materials about the patient, and then repeat some of the tests, adding new ones such as transvenous pth measurement. In the case of the first operation being performed by a less experienced surgeon, I have found that the adenoma is usually in the same old ordinary place on second exploration. When an experienced surgeon misses first time, exotic locations are frequent.
Do specialist surgeons in Utah have the equanimity and wisdom to refer to super-specialist surgeons?