When there are tens and twenty of tanks rolling through a desert, the enemy sees the dust clouds. However, a Bell 47 at high altitude will remain invisible until it moves to be spotted. These aircraft give off noise, and should be heard several kilometers away.
Could be unimportant, could be horribly important.
I imagine something like a "E0 hears helicopter in North Center/North East/East Center/South East/South Center/South West/West Center/North West"