Han Solo thought it was a moon up until he was in tractor beam distance. I’d imagine you wouldn’t see it from hyperspace, but even if you did manage to get within visual distance of it, you’d probably assume it was an exoplanet or asteroid.

I guess the rebels on Alderaan should have been keeping an eye out for this thing but sensors in Star Wars seem to be a funky thing, since they have limited range and ships seem to bypass them all the time.

Edit: Totally forgot - The death star can enter hyperspace as shown in Rogue One. In may have only been in Alderaan’s space for a little while.