Lost Judgment

You get access to the Noise Amp in Chapter 3 as part of the main story. It's during the Side Case 'Dastardly Detective: Introduction'.

What the game counts as 'targets' isn't clear. I had found well over ten targets when this popped, many from side cases.

But what definitely do count are targets found through Squirrel Search, which unlocks early in Chapter 5 (the 'Squirrels Just Wanna Have Fun' Side Case). I thought at first the squirrels didn't count as targets, it was the items at the end of each search. But then this popped for me on a squirrel...

Anyway here are all the squirrel searches that involve the Noise Amp:

Yokohama

• Seiryo High, part of the Side Case 'Squirrels Just Wanna Have Fun'
• Outside Seiryo High, down by the river footpath, near the gate
• On a wall down the street west of the Ebisu Pawn.
• West along the wall from the Jinnai Station Poppo (Interior) on the courthouse steps
• In the courtyard of the big hotel at the east end of Hamakita Ave
• West of Survive Bar
• In the alley just south of the Bar District Taxi
• South end of Sunrise St., on the stairs leading to the water (#1, detector) then north down the stairs on the wall (#2, noise amp)
• Inside the bathroom in the Homeless Camp
• South of Yokohama 99 on the wall near the construction site
• Inside the mouth of the dinosaur outside Wette Kitchen
• Inside Welcome Pharmacy

Kamurocho

• In the midpoint of the Hotel District St
• To the right of the pink banner on the W. Shichifuku Poppo
• Inside Yoshida Batting Center (#1, detector) then in the side hallway behind the standing ashtray (#2, noise amp)
• Inside Kanrai
• On the gate to Kamurocho Hills, near the Park Blvd. Taxi
• In Theater Square on the ground
• On N. Senryo Ave., at the entrance to the alley
• As you enter the Champion District from the west, on a wall
• Inside the little walkway behind a wall at the corner of Nakamichi
• North of Kyushu No.1 Star, next to the shrimp sign on the building
• Inside the smoking area in front of Sushi Gin
• On the back wall of Café Alps (#1, detector) then in the café back room, on the exit sign (#2, noise amp)
• On the ground across from Ebisu Pawn
• On a pillar east of Yagami Detective Agency

Squirrel information compiled from CyricZ's amazing GameFAQs guide (which includes maps of the above):
https://gamefaqs.gamespot.com/ps4/320117-lost-judgment/faqs/...