How can I find my phone when it's on silent/vibrate mode?
If you have a Google account associated with the device, you can use Google Device Manager to send an alert to your phone.


  
  Sign in to your Google Account on android.com/devicemanager.
  If you have more than one device, click the arrow next to the device name to select a different device.
  Select any of the following options in the top left corner of the screen:
  
  
  Ring: Rings your device at full volume for 5 minutes - even if it's set to silent or vibrate.
  Lock: Locks your device with a new password.
  Erase: Permanently deletes all of your data.
  
  


You can also use the Device Manager Android app for this.


  
  Download and install the Android Device Manager app from the Google Play store on your device.
  Open the Device Manager app on your device.
  Sign in to your Google Account.
  If you have more than one device, click the arrow next to the device name to select a different device.
  Select any of the following options in the top left corner of the screen:
  
  
  Ring: Rings your device at full volume for 5 minutes - even if it's set to silent or vibrate.
  Lock: Locks your device with a new password.
  Erase: Permanently deletes all of your data.
  
  


(source: Android Device Manager - Google Help)