General Rule
Enemy units may be known or not depending on the tactical situation on the map. Unknown enemy units are still visible but their values are unknown. There is no invisible unit.
Is this unit a single step security brigade or a full power armored regiment ?
When Units are Known
Units are known when adjacent to an enemy unit. This status is judged at the end of every kind of movement phase. Furthermore, units that engaged in combat are known even if not adjacent to enemy units at the end of the combat phase.