Ah man, that sounds really annoying! Any chance you might find it?
I'm just at the stage now where I'm doing my first flights, and was thinking about experimenting with alt hold, but after reading your story I might wait and set up some proper failsafes.
I was thinking the best one would be the lost radio failsafe, so if it did start flying off, you can just turn off the transmitter and it will just automatically land.
Thanks for telling your story.