Yeah, without the phone owners permission...it *shouldnt* be easy to just hunt down someones phone via the phones GPS. There are apps that do this, but they often require some kinda of prior set-up...
Unless...its an apple device ?. Apple has a 'find my iphone' software service, that might be able to help you. I think you need to log into an apple web page with the apple ID (and password) of the phone, and even then the phone has to be on, have a cell signal, have the GPS on and receiving a signal to get a precise location. I think that this service can also make the phone beep/light up (even if its set to vibrate) so you can find it under a pile of clothes...
edit-nevermind - even that needs to be set up ahead of time:
https://support.apple.com/kb/PH2697?viewlocale=en_US&locale=en_US
Here is where spdrun rolls into the thread and blasts apple. It'd be funny if we hadn't heard it before.
I'm going to assume that LEO have some tools, and my android phone has manufacture 'find my phone'-like functionality forced upon me. If I was a legit owner maybe the local store/vendor could help.
-John