Theres an achievement for finding lonely robot. It would be good if you hadnt completed 100% before 100% of secrets/rooms were visited. Ive read here in forums about:

1. An easter egg in sewer with pizza, TV, skateboard.
2. Level 1: Room were you could change clothes and same room where you find easter egg one.
3. Level 5: with Dr. Hagan as a pink monster you can talk to and kill.
4. Level 6: Room over the left wall. Music change and is turned of if you kill 1 of the 5 robots.
5. Level 6: Lonely robot.
6. Level 6: Secret room under lonely robot. An item not seen before. Dont know what it does.
7. Secret room over level 6 over lonely robot. Easter egg two.
8. MCD Level 8 last easter egg that gives you special weapon.

I would like to see a complete list.