If you have found a device, turn it on and see if there's a message. The owner may have done this:
Use the Find My app to locate a missing device or item - Use the Find My app to locate a missing device or item - Apple Support
"What happens if you activate Mark As Lost
...You can also display a custom message with your phone number on the Lock screen, and keep track of your device's location."
For some iPhones --> Return a lost, locked iPhone, click here --> Return a lost, locked iPhone - Apple Support
Otherwise turn it into authorities responsible for dealing with lost items.
Apple has two related features, Activation Lock ( Activation Lock for iPhone, iPad, and iPod touch - Apple Support ) and Find My ( iCloud - Find My - Apple ), which are designed to prevent anyone else from using an iPhone, iPad, or iPod touch if it's ever lost or stolen. Deactivation of either requires knowing the Apple ID and password that set the feature.