They found Nova this evening. They searched the field a last time and found her in the low lying ground by a creek. Apparently she died there, and without a struggle (there were no signs of thrashing). It's too late for an autopsy, but she said that it did look to be quick, whatever the cause of death was. The owner said that the leaves had covered her and blended with her coat so much that they had just missed her.
She's understandably very, very upset.. but at least she knows.