I've had runner ducks disappear and found them days later sitting on eggs. I've even had runners sit through all 28 days of brooding... but unfortunately they have always destroyed the eggs right at the end, so I don't let them sit any more.
Anyway, it's quite possible that's happened and if so, you need to find her as soon as you possibly can. She is extremely vulnerable to predators right now. I had one on eggs and she was so well concealed I decided to leave her alone and try to hatch them. She was fine for three weeks before the fox finally found her and in the morning I found eggs and feathers. I was able to save the eggs, but I am so sorry I didn't save the mama.
It is also possible that a predator took her. When was she lost? Day? Night? Were the others upset? Some predators, like hawks and owls, will take the ducks without sign. Sometimes even foxes will do this. But usually there is some indication. I would take the time to search high and low for her. They tend to like well-protected areas, under bushes, behind hay bales, etc. Good luck! I hope you find her safe and sound and can move her to a safe location.