This support article has a list of things to do if your iPhone, iPad, or iPod touch is lost or stolen. --> If your iPhone, iPad, or iPod touch is lost or stolen - Apple Support There is a case for NOT doing step #7 where you remove the device from your account. Removing it would disable Activation Lock and make the device usable to a thief or a finder. As long as you set Lost Mode and/or erase the device, it will be not be recognized as a trusted device. This information is recommended by Apple in this thread I lost my iPhone - Apple Community
- The Find My service described in the article is the only recovery feature Apple offers. It must be set up before the device is lost.
Read this support article to learn what you can do if Find My iPhone or Find My iPad isn't enabled on your missing device --> If Find My iPhone or Find My iPad isn't enabled on your missing device - Apple Support