From The TimeSplitters Wiki

An Easter Egg is an unique feature of a level, often small and serving no advantage or gameplay purpose other than it is there to be found. Below is a list of all known Easter Eggs in Arcade mode. As the story mode (paticularly TSFP's) contains many amounts of features, they are not listed here.

TS1

• Chinese
  • The gong in the lobby can be punched or fired upon so that it rings.

TS2

• Chinese
  • The gong in the lobby can be punched or fired upon so that it rings.

• Hangar
  • It is possible to get on top of the box that the crane pulls about.

• Mexican Mission
  • The bell atop the mission can be fired upon so that it rings.
  • The clay pots around the mission are destructible.

TSFP

• Chinese
  • The gong in the lobby can be punched or fired upon so that it rings.

• Disco
  • Pressing the action button when facing the turntables produces some scratching sound bites.
  • Pressing the action button when facing the microphone produces a microphone screech or tap.
  • Pressing the action button when facing/firing at the guitar produces guitar noises.
  • Pressing the action key when facing the drum kit produces drum sounds.
  • Firing at the appropriate drum/cymbal produces the appropriate sound bite.

• Mexican Mission
  • The bell atop the mission can be fired upon so that it rings.
  • The clay pots around the mission are destructible.

• Subway
  • Pressing the action button when facing the middle lever in the control room makes subway trains fly by the station upon the two tracks. They kill any player upon the track, and cannot be shot nor can mines be placed upon them. Such kills with the trains do not count towards the score of the level puller.

• Venice
  • The player is able to take a ride upon the passing boat(s).

• Zeppelin
  • Pressing the action key when facing a control panel makes the missile bomber drop, any players atop the unit fall with it to the sea below.
  • In the Sky Lounges, the walls have little signs next to the ramps up to the Hydrogen Storage and Bomb Release. There is a mistake in the way that the signs are placed. The Bomb Release is in the middle, where the trapdoor is, and the Hydrogen Storage is the two big rooms that are at the end of the blimp (were Red and Blue Base is).