Sonar drones were floating robotic defensive drones that were part of the security at the estate of General Carolina Davis on Vardona. They send our a sonar pulse to detect cloaked units, then launch an EMP at the target, revealing them. The drones were also equipped with a defensive laser.
Sonar drones appear in the Nova Covert Ops level "In the Enemy's Shadow". They wander in patrol routes, and will pulse green circles when they attempt to detect units. If Nova is caught in those circles while cloaked, she will be revealed, and the drone will fire on her. Though weak on their own, sonar drones are usually surrounded by other large groups of enemies.