The drone may fly off the distance for many different reasons, a few of which might be accidental some intentional. For example, if you fix your DIY drone to go in a straighter line, it can, at some stage, fly from the controlling range. The bigger issues are unintentional fly-aways induced by a lost link between the craft and the controller or flawed GPS. In such cases, there is hardly any you can perform to fly in the drone again to safety. The only choice is to switch on the Return to Home failsafe feature, which will securely fly your craft to the original point on the emergency cases.