What is the eyeball icon over the troops in Rome 2 Total War? It is over the troops in battle, and it turns on and off.
The presence of the eye on a unit's banner indicates that unit is not currently visible to any enemy units. When the eyeball icon is absent from a unit's banner, there is at least one enemy unit that can see that unit of yours.
From twel70's Beginner's Guide to Total War Rome 2 on the Romanian Steam Community page, in the Line of Sight (LoS) section. Link
In some older Total War games, units that could hide in forests would be shown as hidden by a small tree icon.