Here are the locations of all Bonus Levels.
Warning!
No tombstone unturned
(Access all bonus levels in Zombies.)
Locations
1) Level 1 – by a house (top centre).
You must pick up either a bazooka or monster potion to break the hedge.
2) Level 9 – by toxic containers (top centre )
Again, use either a monster vial or shoot through.
3) Level 12 – save all cheerleaders and you’ll be taken to the bonus level automatically.
Currently, this bonus level doesn’t count towards accessed maps.
4) Level 17 – behind cob-webs next to the room with a monster vial (top centre)
5) Level 22 – via hidden passage in a laboratory.
There’s also a hidden, powerful flamer next to the bonus level pick-up!
6) Level 33 – in top left corner, in water area.
Thanks to Lord Of War for his great guide, all credit to his effort. you can also read the original guide from Steam Community. enjoy the game.