Update to Lookout's iOS app makes sure that you never leave your iPhone behind
With this feature, your Apple Watch can show a visual on its screen showing how close or far away you are from your iPhone. And you can arrange to have your iPhone make a loud "scream" to help you find it once it drops out of the range of Lookout's Apple Watch app. Both the watch and the handset must be sharing the same Wi-Fi connection. If the worse case scenario happens and you misplace your iPhone, your Apple Watch can reveal to you the last known GPS coordinates of your phone.
These are pretty useful features for the Lookout app since they really can prevent you from losing your iPhone, or finding it once it is gone. Make sure that you have the latest version of Lookout installed on your iOS flavored handset, and load up your Apple Watch with the Lookout app as well. Do this and you won't be worried about leaving your iPhone behind again.
source: Lookout (iOS) via RedmondPie