The problem is, sensors in Trek have always worked exactly as the plot requires. In The Immunity Syndrome
Chekov was able to detect that all life had been wiped out from a star system when the Enterprise
was nowhere near it. On other occasions, they started to scan for lifeforms from orbit.
As to visually seeing the explosion of Ceti Alpha VI at fifteen light-years out, why would they? Do starship main viewers reach that far? What exactly does the "visual range" mentioned in all Treks mean? Also, how much of a "flash" did the explosion generate?
It also works the other way, where the crew see an explosion in real-time from a distance where the light should have taken hours to reach them, and this isn't just due to the viewscreens, as events have been witnessed in a similar way by just looking out the window (from Ten-Forward for example).