Pinging a dead cellphone
If the phone is literally without a power source, than it is not connected to the cellular network and cannot communicate.  However, up until the point where the power goes out, the phone is still communicating with the network, and which cellular towers it was near is recorded.  Since people do not walk very quickly, you can still find a dead phone by asking the network where (very approximately) it was last located.  Chances are it will still be there a few hours later.