Here is the full list of all 26 Shy Cats Hidden Orchestra achievements.
You found the level 6 secret cat
You found the level 3 secret cat
You completed the level 03
You discovered all cats in main menu
You found the level 10 secret cat
You completed the level 10
You found the level 5 secret cat
You completed the level 05
You found the level 4 secret cat
You completed the level 04
You found the level 2 secret cat
You completed the level 02
You found the level 1 secret cat
You completed the level 01
You solved your first mistery
You found your first key
You found your fist cat at a level
You found a cat at the main menu
You found the level 7 secret cat
You found the level 8 secret cat
You found the level 9 secret cat
You completed the level 06
You completed the level 07
You completed the level 08
You completed the level 09
Congratulations! You found them all!