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