Lost Judgment

You get access to the detector in Chapter 4 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 over ten targets when this popped, many from side cases:

• Dastardly Detective: Introduction (dark car)
• Dastardly Detective: The Wandering Bug (woman in red top)
• Dastardly Detective: Grasping Attention (Penguin inside UFO catcher)
• Dastardly Detective: Before the Torch Burns Out (the drone and the old man)
• Big Shame Hunter (Miu's shoes)

But what definitely do count are targets found through Squirrel Search, which unlocks early in Chapter 5. I'm pretty sure that the squirrels themselves aren't targets. Only searches that end with using the detector to find an item count. They are:

Yokohama

• Inside Le Nouveau Hama, on a painting (target: vase)
• On a pillar in West Jinnai Station (target: high on top of a support strut)
• Next to 'Azure', by La Chatte Blanche (target: tree)
• In the Chinatown park inside the gazebo ceiling (target: lion statue's mouth)
• Fountain next to the skaters in Hamakita Park (target: nearby tree)
• In the alley south of the Bar District Taxi (target: clothes outside Hustle Boutique)
• Homeless camp bathroom (target: pair of tyres)
• Tsurukame Bridge Poppo next to counter (target: drinks in back corner)
• Outside Yokohama 99 (target: bushes)
• Inside Plage (target: sofa cushion)

Kamurocho

• Inside the entertainment booth (southeast of Ikinari Steak) on the back of the door (target: bushes)
• Game Center Charles, at the top of the stairs (target: paper lantern)
• Across from Theater Square Smile Burger in window display (target: white lantern)
• Kamuro Theater roof, behind waterfall (target: leafy wall)
• Kamuro Theater basement, on a shutter (target: circular vent)
• Green electrical box in front of Millennium Tower (target: street sign)

Squirrel information compiled from CyricZ's amazing GameFAQs guide:
https://gamefaqs.gamespot.com/ps4/320117-lost-judgment/faqs/...