[Imp]

Think of it as a unit with unlimited vision TI 500 if you like. Vision has no effect on but it must be able to trace a path to the target free of obstructions. So if it could see that far it would have a LOS.
So it cannot see fire through hills buildings.
Other terrain its like a TI unit I think in that can "see" through some wood hexs that a vision 0 unit can't but TI could.
If the helo is hiding behind woods or buildings it does seem to have a better chance of detecting / locking on than none radar stuff as the angle allows it to see over the obstacle
Basic maths the higher or further away from the obstacle the greater chance to see over it.
If you move helo over a woods or building hex so it has to gain height again a good chance of detecting.

So if distant or worse on high ground it has a very good chance of detecting you as the angle means you have left cover.

Hills are your best friend & flying down wooded streams urban roads can be.
Use the terrain no cover dont buy them.
Hilly attack chopper heaven.
No hills woods & buildings provide cover but flying over them can expose you.