If you ever misplace your device – whether it's lost or you think it may have been stolen – you should immediately use Mark As Lost in the Find My app.
Mark As Lost will lock your screen with a passcode and let you display a custom message with your phone number to help you get it back. You can also erase your device remotely if needed. Your custom message will continue to be displayed even after the device has been erased.
While Activation Lock works in the background to make it more difficult for someone else to use or sell your missing device, Lost Mode will clearly display to whoever finds your device that it still belongs to you and can't be reactivated without your password.
