it happens i've hears stories about exactly this.

anyway, to the OP. i have NO idea why you put ur phone in ur backpack... but anyway, its true that its a phone afterall, not wallet or passport.

the only time i put my phone in my backpack is when im going thru the metaldetector. and i put it in the middle, even if it means it'll take me a few mins to retrieve afterwards.

please please please dont put small-sized valuables at backpack....