There should be a iphone cover that blocks the power button

My girlfriend's iPhone was stolen in a subway station. She immediately rushed home to track it in Find My Iphone. However, the phone had been switched off immediately of course.

If there were an iphone cover that cannot be taken off easily and blocks the power button, you could trace your phone for much longer. And who needs the power button anyway? You could of course take it off with a special screwdriver or a small code lock or something.