Lost quadcopter in Tsing Yee last year November...
Found out the root cause today... It was because the GPS+Compass module loosen from the pole in the middle of flight and the direction became 90 to even 180 degree offset and was wrong!!! I flied always in Alt-Hold mode so it was okay initially, but once after I lost video feed, I panic and switched to Return to Home, {banned} things happened and quad flied away (calculation totally wrong!) and into the sea!!!
But at last, I found the real root cause... Rest In Peace!