Cloaked Drone

The Cloaked Drone is an invisible Sentry Drone mini-boss that Samus cannot lock onto, unless she uses the X-Ray Visor (which she can't get at that point without sequence breaking). It is encountered before locating the Power Bomb in the Central Dynamo of the Phazon Mines. The most effective weapon is a charged Wave Beam shot, though if Missiles are plentiful, an easy strategy is simply to use the Wavebuster until the mechanoid has been destroyed. A Super Missile can destroy the Drone in one hit. Even so, it can be quite a tough fight and it is one of the only bosses which cannot be scanned, and is not named in-game. The drone can be fought from the platform that Samus enters the room in, and not necessarily from the bottom area. After Samus defeats it, she traverses through an electrical maze to gain the Power Bomb upgrade.

It should be noted that the Official Nintendo guide states that the robot can be seen with the Thermal Visor. In the game, however, it cannot be seen without the X-Ray Visor.

Metroid Prime
Cloaked Drone

"The Space Pirates made significant strides in their weapon research, including the creation of Cloaked Drones. The flying robots are easy to take down with the Wavebuster, but you must find them first. Use the Thermal Visor to reveal the cloaked foes' locations. Note that Cloaked Drones cannot be scanned.

After you destroy the Cloaked Drone, a small hole will open in the floor."

Metroid Prime/Metroid Fusion (Prima)
"Drop onto the floor, and a cloaked Sentry Drone attacks you. It's impossible to lock on to this enemy. Just watch for its lights, and manually aim with your Wave Beam. Keep blasting as quickly as you can to destroy it. Because this enemy is cloaked, there's no way to scan it.

Once you destroy the Sentry Drone, a panel on the floor opens."