How accurate are x-rays in detecting hairline fractures?

It depend on : The time when x-ray are done, if it done after a week from the fracture 70% to 80%. Also depend on the reader if he have the experience, but if you have suspension you could always splint and follow or could cat scan for that area.

Varies: It varies a little based upon the area of the bone involved. But your implication is correct, there are times that you can have such a small fracture that it does not always show up on X-ray. Both MRI and ct scan are more sensitive in picking up a fracture than X-ray.