A few people have mentioned that their turrets fire at seemingly nothing, even when there is clear sight of enemies. From my experience, what they are firing at is potential dirt mounds that spawn enemies. This is especial noticeable in the ascalon catacombs, explore mode, lovers battle arena option. When you place the turret it will fire at the ground where a dirt mound will spawn, and will continue to fire at that spot long after it is gone.
from my experience they keep shooting at destroyable objects like suplies, tents, tower and another