GRAYHORIZONSHELI Do you actually mean “they found a missing nut” or “they found that a nut is missing”?
I would assume (and surely at this stage of the game, it’s still an assumption) it would be the latter.
I stress again, I am not a pilot, but I am a mech engineer (albeit from a different discipline). As a slight aside, I have used split pins and wire locks on many a track motorcycle in my time, so as well as those two safety measures, there is (as you stated) the correct torque setting for the nut. On top of that, I don’t know if the aviation world use it or not, but on track bikes, it was also common practice to apply a further safeguard in the form of an appropriate thread locking fluid (yes, usually from the Loctite company).