There are a total of 80 costumes that can be found in some of the levels in the game.
If a level contains a hidden costume that you have not yet collected, a textbox with the text
"I sense a Costume hidden somewhere around here..." will appear above the cat.
To unlock the costume you must first find the crack in the wall and click on it multiple times until it breaks and reveals the costume.

The last costume is found in the final bonus level, so if you want to collect them all (unlocks the GLAMOUR-PUSS achievement) you will have to get a 3-star rating on all levels. The guide below shows you how to complete all levels with a 3-star rating.
https://steamhost.cn/steamcommunity_com/sharedfiles/filedetails/?id=890269545

Costume #01

Thief Mask
Found in the living room - level 3

Costume #02

Saucepan
Found in the living room - level 6

Costume #03

Cage
Found in the living room - level 7

Costume #04

Star Sunglasses
Found in the living room - level 9

Costume #05

Groucho Glasses
Found in the living room - level 11

Costume #06

Clown Nose
Found in the living room - level 12

Costume #07

Pig Nose
Found in the living room - level 13

Costume #08

Red Cap
Found in the living room - level 14

Costume #09

Heart Antenna Headband
Found in the living room - level 15

Costume #10

Devil Horns
Found in the living room - level 16

Costume #11

Arrow Through the Head
Found in the living room - level 17

Costume #12

Horned Helmet
Found in the living room - level 19

Costume #13

Surgeon Mask
Found in the living room - level 20

Costume #14

Conical Hat
Found in the living room - level 21

Costume #15

Glasses
Found in the living room - level 23

Costume #16

Beret
Found in the living room - level 24

Costume #17

Gas Mask
Found in the attic - level 2

Costume #18

Skull Mask
Found in the attic - level 3

Costume #19

Grey Beard
Found in the attic - level 4

Costume #20

Rose
Found in the attic - level 6

Costume #21

Baseball Cap & Sunglasses
Found in the attic - level 7

Costume #22

Mr. T Hairstyle
Found in the attic - level 8

Costume #23

Wrestler Mask
Found in the attic - level 9

Costume #24

Pirate Hat
Found in the attic - level 10

Costume #25

Feathered Headdress
Found in the attic - level 11

Costume #26

Monocle & Moustache
Found in the attic - level 12

Costume #27

Dog Mask
Found in the attic - level 14

Costume #28

Pickelhelm
Found in the attic - level 16

Costume #29

Top Hat
Found in the attic - level 17

Costume #30

Amish Beard & Hat
Found in the attic - level 18

Costume #31

Side Cap
Found in the attic - level 21

Costume #32

Officer Hat
Found in the attic - level 23

Costume #33

Red Headband
Found in the laboratory - level 2

Costume #34

Band-aid
Found in the laboratory - level 3

Costume #35

Fez
Found in the laboratory - level 5

Costume #36

Court Wig
Found in the laboratory - level 6

Costume #37

Graduation Cap
Found in the laboratory - level 7

Costume #38

Spartan Helmet
Found in the laboratory - level 9

Costume #39

Kabuto Helmet
Found in the laboratory - level 10

Costume #40

FBI Cap
Found in the laboratory - level 11

Costume #41

Police Helmet
Found in the laboratory - level 12

Costume #42

Deerstalker & Pipe
Found in the laboratory - level 13

Costume #43

Cthulhu Mask
Found in the laboratory - level 15

Costume #44

Rasta Cap
Found in the laboratory - level 17

Costume #45

Flower Wreath
Found in the laboratory - level 18

Costume #46

Afro
Found in the laboratory - level 19

Costume #47

Turban
Found in the laboratory - level 20

Costume #48

Crown
Found in the laboratory - level 23

Costume #49

Chef Hat
Found in the laboratory - level 24

Costume #50

Block Head
Found in the castle - level 2

Costume #51

Dragon Mask
Found in the castle - level 4

Costume #52

Knight's Helmet
Found in the castle - level 5

Costume #53

Wizard Hat
Found in the castle - level 6

Costume #54

Jester Hat
Found in the castle - level 7

Costume #55

Cookie Glasses
Found in the castle - level 9

Costume #56

Bedsheet Ghost
Found in the castle - level 10

Costume #57

Hairnet
Found in the castle - level 11

Costume #58

Propeller Hat
Found in the castle - level 13

Costume #59

Firefighter Helmet
Found in the castle - level 15

Costume #60

Guy Fawkes Mask
Found in the castle - level 16

Costume #61

Face Mask & Towel
Found in the castle - level 17

Costume #62

Paper Bag
Found in the castle - level 18

Costume #63

Headphones
Found in the castle - level 21

Costume #64

Striped Sunglasses
Found in the castle - level 23

Costume #65

Fortune Teller Hat
Found in bonus level 2

Costume #66

Sheriff Hat
Found in bonus level 4

Costume #67

Panda Mask
Found in bonus level 5

Costume #68

Monster Teeth
Found in bonus level 9

Costume #69

Winged Helmet
Found in bonus level 10

Costume #70

Bonnet
Found in bonus level 11

Costume #71

Pumpkin
Found in bonus level 16

Costume #72

Leprechaun Hat
Found in bonus level 17

Costume #73

Witch Hat
Found in bonus level 18

Costume #74

Egyptian Headdress
Found in bonus level 21

Costume #75

Nurse Hat
Found in bonus level 24

Costume #76

Earmuffs
Found in bonus level 25

Costume #77

Scarf
Found in bonus level 26

Costume #78

Santa Hat
Found in bonus level 28

Costume #79

Winter Hat
Found in bonus level 31

Costume #80

Ice Block
Found in bonus level 32