The base at Fort Greely appears to be quite remote, though there is a small town relatively close to the ABM base. If the launch occured during daytime hours, it's possible that it could be witnessed by a number of people. The people who live there are obviously aware of the mission of the base, and most probably have access to network news broadcasts- so it's likely that if they were alerted to the fact that NK had launched the missile, they could be watching the horizon to see if an ABM is launched to intercept it.
I know I would be watching for it.
Frankly, in light of the numerous failures that have plagued the ground-based interceptor during tests- the US should probably stick to the much more successful SM-3's.
But then again, we still can't be sure that the ships in the area are even loaded with the SM-3 missile.
From what I've read recently, it's quite probable that the ships do have the SM-3, because the US has had the foresight to recognise, and prepare for a threat to emerge from North Korea at some point. The missile entered service with the USN more than a year ago, so where else would the new weapons have been deployed, if not to the Sea of Japan?