Find my iphone: how can the thief change the phone name?

Question

Hello.
I have had 3 iphones stolen the last 5 months.
Finally I got a ping from one of the stolen iPhones. I could follow it for almost 2 months. Since I am in a country were the police wont do anything it does not help me that I have name/address of the thief.
But this strange thing happened last day/last ping I got from my stolen iPhone:
Somehow the device is removed from my "find my iphone" list and/or the device have changed name.
The device was locked with passcode/touch ID.
I have changed password AppleID 3 times since the phone was stolen (well: The police wanted my password/AppleID to track the phone. Somehow the thief got this information and I found out when my address book had 150 new entries from the thief).
I managed to change password and lock the phone remotely after that incident.
To bad I cant ask Apple any question.
How can a device be removed from my account? (I have not got any warnings that someone have logged into my iCloud account from a new device/location)
How can a thief change the name of a stolen iPhone that is locked with touchID/Password/passcode? (Iphone6).  Is there still DFI attacks that can wipe the phones?
How can I check if my account is hacked?
Why cant I remotely detonate my iPhone? The battery would be a nice little bomb making the phone worthless?
(and F lazy Russian tourists police that just ask "what should we do" when I provide address/name/video of a person that have stolen stuff worth over 5000 dollars including iphones/Ipads)

Answer 1

Hi Brirendine,
Take a look at the instructions below for what to do when you sell your iPhone.  If you have followed all of these steps, the new owner should contact his/her carrier to activate the phone.
iCloud: Activation Lock
http://support.apple.com/kb/PH13695
With iOS 7 or later, Find My iPhone includes a feature called Activation Lock, which is turned on automatically when you set up Find My iPhone. Activation Lock makes it harder for anyone to use or sell your iPhone, iPad, or iPod touch if it’s ever lost or stolen. 
With Activation Lock, your Apple ID and password are required before anyone can:
Turn off Find My iPhone on your device
Sign out of iCloud on your device
Erase and reactivate your device
Important:    Make sure to remember your Apple ID and password, and that your password is unique and secure—someone else shouldn’t be able to guess it. For more information, see the Apple Support article Security and your Apple ID. 
If you want to give away or sell your device, be sure to erase your content and settings (in Settings > General > Reset > Erase All Content and Settings). When you erase your content, Find My iPhone and Activation Lock are also turned off. 
If you no longer have the device, follow the instructions to remove a device you no longer have. For more information, see the Apple Support article What to do before selling or giving away your iPhone, iPad, or iPod touch. 
If you’re purchasing or selling a used device, you can check its Activation Lock status at icloud.com/activationlock from a Mac or PC. 
For more information about Activation Lock, see the Apple Support article Find My iPhone Activation Lock. 
Last Modified: Jan 8, 2015
I hope this information helps ....
- Judy