This won't help you find it now because the battery is dead, but for the future I use an app called Where's my Droid.
One of the neat features it has is when your phone gets to 4% battery remaining the phone will send you an email as to where it is located using GPS. Then when the battery dies and the phone is lost you have a place to start looking.
You can try to find the last known location of the phone via Android Device Manager. Log into your Google account and go to ADM.