Hi , I'm James DVM MRCVS and would like to help you with your question
Could you be more specific as to what is your questions . are you asking what could have led to her death?
this is indeed a more difficult case to diagnose without doing a post mortem exam
You mentioned that your other owl has showed no symptoms, this still can not rule out bacterial or parasitical infection.
some individuals are more resistant then others (like in people) and will not be affected by disease.
it might be as well that Elmo was born with some kind of congenital disease , like organs or a metabolic system which is not functioning properly and that over time have led to it's untimely death
another possibility is that Elmo was effected by some feed material , either poisonous or perhaps had developed a sensitivity to a regular feed material which lead to a sever reaction by her body , eventually causing her death.
Sadly , its hard to tell by her presenting symptoms and only a full exam would deliver us with this wanted information .
Sorry I could not have been of more help on the matter.
please, if you have any more questions , feel free to ask