I remember in SINS 1 that we always got audible warnings (as well as an indicator) that an enemy fleet was spotted heading towards a planet and gave us a few moments to "prepare" for their arrival. So far in SINS 2, there is no warning unless you happen to be monitoring the space lanes and see them flying towards your planet. Most of the time it's just "surprise"!
I don't like surprises.
Now I always "assume" that I could be attacked anywhere and try to build defenses as I can afford them, but even so it would be nice to have that brief warning back.