Find my Iphone has been turned off after it has been stolen

Good Evening, I am after some advice...

On Saturday night my Iphone 6 was stolen. I was in Barcelona, Pic-Pocketed (Very common by the sounds of it). Someone told me who had stolen it but they'd ran off through the busy alleys and had vanished.

That night I went online and locked the phone on the find my iphone app (using a MAC computer), put a message and number in etc

Sunday night my friend got a message saying the phone had been switched on with a location, when i tried to check the location on the website but it was very slow and wouldn't actually pin point the location of the phone. I tried it on the MAC computer and there was an issue logging in, it was asking for my security questions which i got wrong and wouldn't let me log in.

Iv spoken to apple support today, reset my security questions, had a chat and according to them theres no device registered to find my iphone on my account. The IMEI of the phone isnt registered switch on on find my iphone so it cant be locked. the chap i spoke to said it would be imposible for them to switch it off without accessing my account, theirs apparently been no strange activity, no attempts at changing email address/passwords. Even tho im now in my icloud account and iv got the email showing that my Iphone was locked, then another showing that it has been found with an exact location, is there anything that can be done now to lock it? Iv got the IMEI and proof of purchase

How is it possible for someone to have switched off the find my iphone? Has someone accessed my account and switch the app off? Or is there another way? Is there anything I can do now?

Thanks for advance