Lost Judgment

Squirrel Search unlocks in Chapter 5 through the Side Case 'Squirrels Just Wanna Have Fun' (near Seiryo High).

You'll need to find 55 squirrels in Yokohama and Kamuroch and use your tools to find an item for each. Then you'll need to complete a short Side Case to get the last squirrel and the achievement.

When you spot a squirrel, use observation mode on it and you'll get a hint as to which tool to use. Sometimes you'll need to solve a sequence of squirrels to get to your loot.

Below are all the squirrel locations and the tools to use, courtesy of CryicZ's magnificent guide on GameFAQs:
https://gamefaqs.gamespot.com/ps4/320117-lost-judgment/faqs/...
(there are maps here too if you're struggling to find a squirrel.)

Yokohama

1. Seiryo High, part of the Side Case - Noise Amp
2. (#1) Outside Seiryo High, down by the river footpath, near the gate - Noise Amp. (#2) On the bridge support over the river. Pointing down - Drone
3. On the little river walk southwest of the Batting Center, near the water - Dog
4. Inside Le nouveau hama, in one of the paintings on the wall - Detector leads to nearby flower vase.
5. In the West Jinnai Station pavilion, on one of the pillars - Detector leads to a support strut to the north. Look up.
6. (#1) On a wall down the street west of the Ebisu Pawn - Noise Amp. (#2) Further up to the right. Pointing right - drone
7. Right next to the Carriage Highway Taxi Stand - Dog
8. (#1) West along the wall from the Jinnai Station Poppo (Interior) - Noise Amp. (#2) North on the courthouse steps - Detector. (#3) On the notice board to the east - Dog
9. North of Coming Boon, in the tiles on the sidewalk - Dog
10. In the sunken walkway near the business called "Azure" - Detector points to the nearby tree, in a knothole.
11. (#1) Northeast corner of the café at the west end of Hamakita Park - Detector. (#2) Backside of the café. Pointing up - Drone
12. On a tree north of the Hamakita Park Ave. sky bridge - Dog
13. In the park in Chinatown on the east side. Inside the gazebo ceiling - Detector leads to the statue's mouth.
14. In the courtyard of the big hotel at the east end of Hamakita Ave - Noise Amp
15. In the fountain next to the Skaters - Detector
16. To the west of Survive Bar - Noise Amp
17. (#1) In the alley just south of the Bar District Taxi - Noise Amp. (#2) Further in the alley - Detector points to the rack of clothes outside Hustle Boutique.
18. (#1) In the south end of the Bar District on the west wall - Detector. (#2) To the north, the recessed wall on the building. Pointing down - Drone
19. (#1) South end of Sunrise St., on the stairs leading to the water - Detector. (#2) Just north down the stairs on the wall - Noise Amp. (#3) On the bridge's outer wall to the east. Pointing up - Drone
20. (#1) Inside the bathroom in the Homeless Camp - Noise Amp. (#2) On a blue tarp shelter on the east side of the camp - Detector points to a pair of tires at the south end of the camp.
21. Inside the Poppo, next to the counter - Detector points you to the drink shelf in the back corner.
22. (#1) In the alley east of Hyakkei St. Entrance Taxi - Detector. (#2) North of the Hyakkei St. Entrance Taxi. Pointing down - Drone
23. South of the Hello Work Taxi, on the south wall - Dog
24. (#1) South of Yokohama 99 on the wall near the construction site - Noise Amp. (#2) The telephone pole to the south - Detector. (#3) Another telephone pole to the south. Pointing up - Drone
25. Just outside Yokohama 99 - Detector points to the bushes to the southwest.
26. Inside Plage, on the south wall - Detector points to the pillow on the couch.
27. Inside the mouth of the dinosaur outside Wette Kitchen. - Noise Amp
28. Inside Welcome Pharmacy. Turn right when entering and look in the corner - Noise Amp
29. On the east wall in Ijin St - Dog

Kamurocho

1. In the midpoint of the Hotel District St - Noise Amp
2. (#1) On the wall at the east end of the Hotel District St - Detector. (#2) On a wall of the hotel to the southwest. Pointing down - Drone
3. (#1) To the right of the pink banner on the W. Shichifuku Poppo - Noise Amp. (#2) To the right of that one, a little higher on the wall - Drone
4. (#1) Inside Yoshida Batting Center on the south wall - Detector. (#2) In the side hallway behind the standing ashtray - Noise Amp
5. Inside the MEB (southeast of Ikinari Steak) on the back of the door - Detector points to a corner of the bushes across the street.
6. Inside Kanrai, on the folding screen near the reception area - Noise Amp
7. (#1) In the Shichifuku Parking Lot, the back of the orange ticket kiosk - Detector. (#2) Backside of the parking rates sign to the north. Pointing up - Drone
8. In Game Center Charles, on the wall at the top of the stairs - Detector points outside to the paper lantern.
9. On the gate to Kamurocho Hills, near the Park Blvd. Taxi. - Noise Amp
10. Across the street from the Smile Burger, in the window display - Detector points to the white lantern above the blue vending machine.
11. In Theater Square right on the ground - Noise Amp
12. To the east of the Theater Square Club SEGA, left of NY Hot Dog - Dog
13. On the roof of the Kamuro Theater, in the NW corner behind a waterfall - Detector points to southwest, on a leafy wall.
14. Basement of Kamuro Theater, on a shutter near Millennium Tower - Detector points east and to a circular vent attached to the floor above.
15. On N. Senryo Ave., at the entrance to the alley from Chapter 1 - Noise Amp
16. (#1) As you enter the Champion District from the west, on a wall - Noise Amp. (#2) On the north side of the Champion District, on a bar wall - Detector. (#3) On a set of stairs in the center of the district - Dog
17. In Tenkaichi Alley, on the east side of the Jungle Boy wall - Dog
18. Inside the little walkway behind a wall at the corner of Nakamichi - Noise Amp
19. On the green electrical box near the street in front of Millennium Tower - Detector points to the street sign to the northeast.
20. In the tiny lot to the east of Sushi Zanmai - Dog
21. North of Kyushu No.1 Star, next to the shrimp sign on the building - Noise Amp
22. On the ground in front of the Tenkaichi St. entrance - Dog
23. Inside the smoking area in front of Sushi Gin - Noise Amp
24. (#1) On the back wall of Café Alps - Detector. (#2) In the café back room, on the exit sign - Noise Amp
25. (#1) On the ground across from Ebisu Pawn - Noise Amp. (#2) In the alley near Ebisu Pawn - Dog
26. (#1) On a gray pillar to the east of Yagami Detective Agency - Noise Amp. (#2) Above the first squirrel. Pointing down - Drone

Once you’ve collected 55, head to Fukutoku Park in Yokohama for the Side Case 'Big Squirrels Don't Cry'. Complete this (the squirrel is on the mound) and the achievement should pop.