So, my shiny Nighthawk Pro went missing back at the start of October on its first trip out since getting a new arm, required from a crash in its first week. Today, four and a half months later, I found the quad. Looks in pretty good condition - not even a bent prop or aerial, but most of the metal seems to have rusted - especially the bolts and nuts, and even my shiny new Mobius which was having its maiden outing. I haven't tried it out yet, but I think the motors may be too rusty to work properly as they're quite stiff to the touch.
Anyway, it seems that the problem was just that the battery plug popped out of the socket on the main board. Apart from that being disconnected it was all fine (although the battery alarm also seemed to have been missing too, although you can hear it beeping just after the crash). Still, the SD card in the Mobius was fine. Here's the flight: