No, it's not about Area 51.
One of your soldier has spotted an alien.
There is a red square to the right of the screen when the soldier is selected, and he can see the alien.
So, your soldier can see the alien, but you can't see the alien on the screen.
This happens when the alien is located in a dark part of the screen.
You know, when you move the mouse cursor all over the screen to try to determine where is located the alien.
Yes? You get it!
So, I suggest to display a yellow 3D box at the alien position when this happens; the 3D box is removed as soon as the alien sprite is clearly visible on the map.