What Lost Mode does
You can use Find My to mark an eligible missing device as lost, display contact information, and take other actions available for that device. Find My must have been enabled before the device went missing. See Apple’s Lost Mode instructions.
What Activation Lock does
Activation Lock is tied to Find My and the owner’s Apple Account. It can remain associated with a device after it is erased and appear when someone tries to activate it. It is an ownership protection, not a message displayed to help someone return a device.
If you found a device
Do not try to access the owner’s account or remove the device from Find My. Follow any contact instructions shown on the screen or hand it to an appropriate local lost-property service.
If your own device is missing
Use Find My to mark it as lost as soon as possible, and protect your Apple Account. Be wary of unsolicited messages claiming someone found your device and asking for a passcode or verification code.