Don't beat yourself up about it mate, despite what others may say the majority of folk would have done the same thing as factory resetting the box never caused any issues in the past and it's debatable that it's even related to the missing netid problem. I'd certainly done it numerous times without any problems.
I've been asking about on a few forums and there's a few theories but no solutions at present I'm afraid. Previously when Uddingston disappeared it then re-appeared (with another 7 missing netids) and I and other folk managed to pick it up after factory resetting and rescanning as normal with none of the workarounds (adding frequencies etc). Some have mentioned it might be superhub related but I'm not sure as the hub isn't connected when I'm scanning, unless it can "lock" or "hide" the netid even when its not connected. Who knows. Personally I think it has to do with the firmware and scanning process on the a850 as it is different to the a780 which has no issues.
If I were you i'd just keep factory resetting and rescanning. You might notice that you get a different number of netids each time (I noticed this in the past) and it might reappear.
Good luck and let us know how you get on!