Well given FTL sensors, i think people might notice if the Mutura Nebula suddenly disappeared. Abd what's this new planet that was never there beofre. And you say we can't go to the mutura sector anymore.
You're assuming that everyone in the twenty fourth century is a space-traveler who's aware of every astrological phenomenon. I don't know if that's the case. Do people today know about every weather disturbance or natural disaster? The major events (severe tornadoes, hurricanes) get attention, but if a lightning strike knocks over a tree on the east side of Indianapolis, who besides people in the area know about it or even care?
The Mutara Nebula seems to be located near Klingon space. As the Klingons were enemies of the Federation during the time of the Genesis fiasco, it's possible that most Federation citizens avoided the area even before Genesis was formed. The planet's sudden existence might have gotten attention from civilian scientists and astronomers who had no direct ties to Starfleet or the Federation's science division, but can we really expect that they'd have gotten the word out about the planet by posting information on a blog? If Starfleet was really serious about keeping Genesis under wraps, it's likely they'd have had any such material taken down before it had a chance to become disseminated.