Lost your phone and now you don't know where to find it.? Don't worry because Google's making it even easier to track down your missing android mobile device. While Android Device Manager already exists to help find lost phones and tablets, just getting to your account's ADM menu often requires a search of its own. But Find My Phone is as simple as a web query. In fact, that's literally what you're doing. Simply open a Google omnibox and type "find my phone" to display a map that reveals the phone's current resting place. If the device is nearby, you can also opt to ring your cell phone even if it was put on silence the phone will still ring loud to tell you its current location. Unfortunately, Find My Phone does not allow you to lock or erase your device should it be stolen or, say, left at the local pub -- you will still need Android Device Manager to do that.
Here's the Google's original post that explains it.