My son misplaced his phone. I assumed he was safe because it had a PIN on it and it was locked. Turns out someone must have found it and was using it to rack up big long distance phone bills. Here’s just one way people can do this: 3 Ways to Bypass iPhone Passcode – wikiHow.
Obviously I am not encouraging people to do this, any more than I would encourage people to steal or commit other crimes. I am encouraging you to call your provider whenever you can’t find your phone.