Been discussed before, what happened, happened, heh.
But I agree with you, since people of the 29th century and on can scan time with sensors like sensors scan space, they should have seen instantly when the Narada went into the past, and went to investigate.
However, one reason they might not have is because it was a natural
event in the timeline, and not a synthetic event.
For example, when Voyager went to 1996, it was an accident caused by a future time agent failing in his job to destroy Voyager, and they were pulled into the past.
When Spock destroyed the supernova with red matter, that created a time rift, but that was always meant to happen, or something, I dunno, now I'm confused.