The "hollow arrow" icon shows iPhone users that "Geofencing" is enabled and currently being used.
Geofencing creates a virtual fence around a location on a map, and if the iPhone enters or leaves that area the device will perform an action, such as sending an alert or data. It’s a powerful feature that was introduced in iOS 5, but there are currently only a few apps that take advantage of it. Lookout uses Geofencing to increase the accuracy of Locates, and also to aid in location if the Lookout app is closed.
**Please note** The icon will only appear on iOS 5, 6 & 7