hey andy. glad you skated on this fly away. I read your story and all the little details you put in there so this will not repeat what you or anyone else said.
3 things. the compass heading indicator, shows the front of the aircraft and the camera direction in the lower left on the app. people have reported this being reversed on a power up and sometime after takeoff the aircraft grows a mind of its own and just flys away, usually crashes bad. what is happening is the compass heading has reversed 180 degrees(reasons unknown) and does the opposite of what you tell it to do. it's a very easy thing to miss and I personally never gave it a second thought till I read a few people had proof of this happening, now I check it every time. so while your trying to tell it to fly back to you your actually telling it to go the opposite way, when the fly away occurred, did you try telling the aircraft to fly away from you or did you go full hands off and just watch it do it's own thing. also,
were you in function mode? that mode can be really confusing easily and in a panic situation most people will not be able to fly in any direction they want and that switch is so stupidly small its real easy to think your in atti mode when your really not.
3rd hypothesis. on the map screen you see 2 dots, red and green. I believe one is the return to home point and the second is you. sometimes these can appear WAY WAY off. they are suppose to reset when you arm the motors but I have seen many times they do not and can relocate on their own. sometimes they are miles away from your actual location, sometimes where it was last turned on and the points never reset.
the inspire is hard wired in the firmware to pretty much lock you out and fly to one of these home points when the battery voltage level gets too low. it's actually a really bad idea that has been causing more crashes then it's prevented. this sounds a lot like what happened to you if they were really far away then it would make sense it just fly off and landed at over 50% battery life, it would take a lot of time to fly 1/2 mile away on auto pilot.