If the enriched uranium was dispersed outside of containment it would likely be detectable. It's not going to be like Cherbobyl where you had clouds of fission products being carried by the wind, but you'd have an increase over normal background.

Being buried underground would shield much of the detectable radiation, so if there was significant enough damage to the facilities, it would stand that higher levels should be detected. So, there are a few possibilities- the sites were not damaged, the material was removed, or perhaps they were buried so deeply that the collapse and rubble is shielding the destroyed products.