In TacOps, units only fire direct fire when they have a technically reasonable chance of killing a vehicle or killing or suppressing a dismounted infantry unit at the range, angle, and situation of the instant. For example, a T55 tank will not fire against the frontal armor of an M1 tank except perhaps at point blank range.
In every direct fire engagement by a given unit marker, there is also a low random chance that one or more or all of the teams/squads or one or more or all of the vehicles in the marker will not fire even when technically qualified to do so. The probablility of this happening increases in rough terrain.
It can also happen that enemy markers which are still shown to the player on his screen may not still be actually visible to any of his units. A unit marker may sometimes be shown on the screen slightly after it is no longer actually visible to any enemy units.