There are many secrets to find in the zombie maps. I've put all these here:-

Nacht Der Untoten

Hidden Song - Shoot/Knife the Radio in the random box room.

Verrück

Hidden Song - Flush the toilet three times (X).

Scary Sounds in the morgue if you get close enough.

Also the chair in the morgue - if you press (X) a few times, you get a drill sfx.

Shi No Numa

Hidden Song - Press X on the phone

Der Reise
Hidden Song - Press X on the 3 glowing brains in jars (2 in the lab, 1 in teleporter room with the vats).

Radios - 5 Radio's to find. Press X for some background intel.
1 - Start location
2 - Furnace under lab teleporter
3 - Room just in from double tap machine
4 - By teleporter with swastika
5 - Pathway under the quick revive room

Elevate your senses achievement - once you have pack a punched a gun, go towards the the lab from the pack a punch machine and on first external window, shoot a switch in the distance. You now start an easter egg game to find the teddy bears.

YouTube videos of these can be found on our Easter Egg Video page.