The question is not about the body - the police do not resurrect (as of now), but about its condition and the consequences of the discovery:
First, if the body looks fresh and recent - you must call the ambulance first, as the person might be not as much dead as you think!
If it's fresh there might be a following Pikuach Nefesh danger for others (think of a serial killer) or for you to be found near the body (if the killer is near). In this case, you must call the police.
If the body is old and there's no reason to believe that an immediate danger emerges for you, for the body or for others, there's no permission to break Shabbat.
PS on #2: I came across (what looked to me like) a dead homeless person some 12 years ago in Jerusalem, and I ran to a secular guy to use his phone to call an ambulance. Turned out he passed out and it saved his life!