Stolen phone

Do's, If you had enabled "Find My" before it was lost

• Mark As Lost.
• Erase your device. If your missing device is covered by AppleCare+ with Theft and Loss, you shouldn't erase your device. Instead, file a claim for your lost or stolen device.
• 

Device Offline?

I see my device in Find My, but it's offline

If you see "Offline," "No location available," or "Location Services Off," your device might be offline for one of these reasons:

Don't see your device?

• You might not have Find My turned on.
• If you're using the Find My app, you can't see your device's location if more than 7 days have passed since its last location was sent to Apple.
• If you're using iCloud.com/find, you can't see your device's location if it's powered off, the battery has run out, or more than 24 hours has passed since its last location was sent to Apple.
• Learn how you can still protect your information.

Don'ts

If the device is removed from an account after it is erased, Activation Lock will be turned off. This allows another person to turn on and use the device.

An example of my iPhone 5 is given in the GIF below. There is no "Undo" after you remove it from your account.

Beware of Phishing!

Be careful and be smart to --> Recognize and avoid phishing messages, phoney support calls ...

Note: Apple will never send you any info on Lost/Found iPhones. It (Phishing) may appear as the image given below.

What to do if your phone is lost or stolen.

If your iPhone, iPad, or iPod touch is lost or stolen - Apple Support

No one can open it unless they have the device passcode. It is impossible.