Found a film or strip stuck inside the back cover of Lenovo K3 Note
According to people at https://www.reddit.com/r/lenovok3note/comments/3eousk/could_the_lenovo_k3_note_be_optimised_for/ it's used for shielding the Sim cards. 

I'd be surprised, and posit it is actually an anti-theft or tracking RFID sticker - possibly bigger then typical as it needs to work through more packaging?