http://www.livescience.com/53816-large-ish-meteor-hits-earth-but-no-one-notices.html?li_source=LI&li_medium=more-from-livescienceNormally, atmospheric explosions are recorded by seismic monitors, microphones and/or satellite observations. As Plait pointed out, as the impact was in open ocean, it's not likely that seismic monitors would have been used to record the impact energy. As the military has pretty obvious reasons for monitoring atmospheric explosions, it seems likely the data came from a classified source, probably satellites.
The first thing that came to my mind were ocean tsunami buoys. If a large meteor were to hit the ocean, I immediately envision a tidal wave of some size to be picked up by at least one buoy. Yet, the media missed it, even if they were to pen an article on another page about said buoys unrelated to meteors, nobody on staff would've be wise enough to connect the two.
