*Apparently* emergency services have a way to over ride your code, but I dunno how true that is. Interesting question and I've wondered the same thing. Only recently I put a code on mine to stop my kids prank people and playing games!
DH is a paramedic in nsw, and he doesn't have any clue how to access people's phones unless it's the way I described and they have ICE contacts setup. I can't speak for police or firies or other states though.