Observation ship of the Hierarchy, whose scanner accidentally tapped into the Doctor's daydreams and led its crew to fear Voyager in Tinker, Tenor, Doctor, Spy.
Hierarchy ships surveyed passing vessels from hiding to decide whether they were worth attacking. In Tinker, Tenor, Doctor, Spy, a Hierarchy observation ship watched Voyager from a nebula, and since normal scans failed, one of its crew used a tunneling scan that latched on to the Doctor's program.
The Doctor had been daydreaming about commanding the ship and firing a mighty photonic cannon, and the Hierarchy took his fantasies for reality. Afraid of punishment, the observer contacted the Doctor, who then had to act out his daydream for real to frighten the Hierarchy away.