Perfect Dark/Unused Content By Level/Area 51

Keyhole
The duct required to enter the showers in Rescue on Perfect Agent, which contains the map's cheese, actually has a second Easter Egg at the end of it that can't be seen normally since there is no way to get past the first drop into the locker room. The duct actually continues quite a distance to the left past the corner that can be seen normally. There are vents in the floor looking into the showers. It then turns left again, with more vents in the floor looking into the locker room, and at the end of this stretch of duct is a wall with a keyhole carved out of it. Beyond the keyhole is another tiny section of duct with bright yellow lighting. While it's possible to look up into the duct through the locker room vents, the keyhole can only be seen with the X-Ray Scanner equipped or through the FarSight's scope, both of which prevent seeing its intended lighting.

The moon jump GameShark code in the Infiltration section can also be used to get into the inaccessible parts of this duct and see this keyhole. Interestingly, there is collision at the end of the last vent before the keyhole preventing the player from getting any closer to it than shown in the screenshot.

A51 Faceplate
While this helmeted, masked head is available to use on characters in the Combat Simulator, its file name proves it was originally intended to be utilized in Area 51.

A51 Guard
An alternate model for the Area 51 Guard. While the body is identical to the final model, the helmeted head is modeled as a part of it. The helmet is the same as the one worn by the A51 Faceplate head, but without the faceplate, obviously. The head itself is unique; it appears to be Karl Hilton, who left Rare during Perfect Dark's development, which would explain why it wasn't used as one of the randomized heads.

A51 Chair
A silver swivel chair with black cushions. These fit in really good thematically, but all of the chairs used in Area 51 are the plain black ones from the dataDyne building.

A51 Screen
A silver monitor which displays an image of a desktop. Strangely, all of the many tables in Area 51 are devoid of PCs or other props, so this would've fit great. It is not set up to work as a screened object, so it can only display the desktop image.

Unused Pads
There are pads for an elevator platform in the large room with the duct that leads to the showers in Rescue. It really serves no purpose in the final layout of this room since there is a ramp connecting the same floor and walkway it would have.

There are pads for benches of some sort in the locker room.

Mine Sign
A chunk of Area 51 exterior-style wall with an upward facing light, which illuminates a silly looking skull and crossbones. It was intended for the minefield, but there's no pad for it there.

A51 Radar Console
A wall mounted tan console with an oval radar display and four orange lights. Joanna has to shut down the air intercept radar in this mission, but this console isn't used in the bunker and there isn't a pad for it.

Unused Pads
Pads hint that the guard tower may have originally been accessed via an elevator, rather than a ladder.

Unused Dialogue
Using a moon jump GameShark code, it is possible to hear two pieces of unused speech from Carrington.

To jump up, hold the B button on controller 1 when Joanna's height is changing, such as when going over an incline or falling from a ledge. Release the B button to fall.

To hear the first line, jump in the area with the lift doors to the hangar. Carrington will buzz in and say "The air intercept radar is controlled from that bunker."

To hear the second, place the explosives in the bunker and blow it up as normal. Jump in the same area as before and Carrington will buzz in once again to say "The hangar lift is the other side of the huge door." This line will not play if the air intercept radar has not been shut down.

The action blocks which control these two lines seem to be using placeholder values, both triggering when Joanna is in room, which is above the area with the lifts and bunker.

Unused Text
The outro has a few extra lines.

Lab Technician: What the hell do you think you're doing? This is supposed to be a sealed room! Don't you know anything about autopsy procedures?

Lab Technician: Director Easton will hear about this, young lady.

Joanna: Of course he's going to hear of it, you moron. I've just got into his top secret base, and now I'm stealing his alien. No audio exists for Joanna's line. The lab tech's lines are used in Escape's intro instead, with a new line for Joanna.

Mystery Objects
Bizarrely, there is an invisible, collisionless tablejust in front of the door in the starting room. It has a tag, but nothing makes use of it. Its pad makes it float a slight distance above the ground, but it has the fall to ground property set.

There is another hoverbed with a Maian on it and a hovercrate in the second autopsy room. Unlike most objects in placed in inaccessible locations, these are never used by the mission. They can actually be seen normally by choosing to escape from Area 51 on the hoverbike. You cannot enter the autopsy room itself, but can look through the window of the locked doors.

Unused Text
Alien medpack operation aborted. It is not possible to abort administering the alien medpack once it is activated, so this goes unused.

Outer hangar door is closing. Inner hangar door is closing. It's also not possible to close the hangar doors once they've been opened.

It's dangerous, Jo... I can handle it... Trust me. ...Okay. I'll cover the stairs - you take the ramp. Four extra lines for Joanna and Jonathan, discussing who would escape on the hoverbike.

Please stop! I'm not supposed to die. She... Arggghhh! Found after the lab tech's "that wasn't meant to happen" line. No audio exists for it.

Gasps, chokes, and wheezes. Audio does exist for these, but the text string isn't shown.

Technological Regressions
Maian SOS reuses many of its object placements from Rescue and Escape, but common objects are used instead of the Area 51 specific ones used in the other missions.

The silver wall mounted autogun model is replaced with the blue "roof gun" from GoldenEye, which doesn't even attach to the ceiling where it is placed properly, since the pad is intended for a wall mounted gun. The blue terminals from the dataDyne building are used instead of the black ones seen in Rescue.

The Area 51 barrels, likewise, are replaced with the ones from Chicago. This is also the case in Rescue, but the barrels can only normally be seen with the X-Ray Scanner since they're all behind locked doors.