Sensor drones were quadrupedal robotic defense drones used to defend the Davis's compound of General Carolina Davis on Vardona. They wandered the compound and its sewers attempting to detect cloaked threats. When a threat was detected, the drone would explode in an electronic pulse, damaging the threat and revealing it.
Sensor drones appear in the Nova Covert Ops level "In the Enemy's Shadow". They wander in patrol routes with a red circle showing their detection range. If Nova wanders into their radius, the sensor drone explodes, dealing 100 damage and revealing Nova if she is cloaked. Their detection range is small however, and they can be used against the Defenders of Man with dominate.
The sensor appears to use a model similar to the canceled shredder unit.