Just like Perfect Dark 64’s hidden cheese, Perfect Dark Zero contains hidden rats. A rat is located in each level and acts as simply an easter egg. Rats are usually in shadows or in spots where it would be tortured.

Mission 00: Datacore | Demolition

After you blow up the CPU with your Cam-Spy you will go down a hallway and enter the room with the scientist (the one where you shoot the spider bots). Upon entering, head straight towards the railing and look at the floor. Through this glass floor you will notice a rat with bubbles coming off of it.

Mission 01: Nightclub | Stakeout

Once you kill the first two guards, go to the main staircase on the left. The rat is just lying at the bottom of the fountain.

Mission 02: Subway | Retrieval

Location not known

Mission 03: Rooftops | Escape

Once you get to the ammo bow with a DW-P5 inside of it opened, the rat is directly behind it in the shadows.

Mission 04: Mansion | Infiltration

The rat is found at the left snow mound at end of level near the pond and arena entrance. Its head is buried in the snow.

Mission 05: Laboratory | Rescue

The rat is on a testing machine directly next to the silver keycard.

Mission 06: River | Extraction

Location not known

Mission 07: Trinity | Infiltration

A sniper rifle is recommended. After you defeat the brothers, turn around and go to the left staircase. Once you are down there, zoom into the water and look for a submarine. Once you find it, look on the top, near the entrance hatch. Look to the left and the rat should be there. A second rat is on the launch pad. It is smoking.

Mission 08: Trinity | Escape

Location not known.

Mission 09: Jungle | Storm

Location not known.

Mission 10: Temple | Surveillance

The rat is impaled on one of the spikes in the first trap. #2 on the floor on the left side of the room where the statue heads breathe fire; there’s a smouldering rat sitting there.

Mission 11: Outpost | Rescue

The rat is hidden behind the first staircase you go down at the beginning of the level. It is hidden behind a box.

Mission 12: Arena | Showdown

Location not known.