Well, if you have an iPhone, use findmyiphone.
otherwise, retrace steps.
If those don't work, you could try getting the IP of the phone, hacking into the OS, turning on the GPS, and get the GPS location of the device. I believe that laws define hacking as illegally bypassing authentication into a system you are not permitted, as the phone is your own, you give, or deny, yourself permission into the phone. So, it would be legal, and defined as ethical-hacking, which is entirely legal.