The enemy units are not in visual range. You are looking at the intelligence view gathered by your spies or planes on patrol.

The unit description shows when the data was gathered / updated last.
If you move a unit closer you can get a current view of the enemy, but keep in mind that ONLY a patrol from a plane can update the intel image. So if you move back out of sight again the intel image will be displayed again.
im not 15years old to sell this