Thanks for all your replies. She has not exhibited any noticeable signs of illness or injury. She has been going to a nest (too old to lay eggs anymore) every morning for a couple of months and sitting there for a few hours, which a couple of my other ducks historically have done. The last week or so she would disappear for periods of time but show up for the afternoon feeding. Sort of acting broody like she did a couple of times when younger. She wasn't around for the afternoon feeding yesterday and I thought she would eventually show up - but she didn't. We looked in the usual places she would go or has gone over the years but not there. No visible signs (i.e. feathers or blood) to indicate a predator. So she either wandered off or perhaps a bald eagle took here as there is a pair in the area we see now and then. But her remaining companion didn't act as though there was a traumatic ending. So I am now down to one Khaki Campbell duck with Tildy's disappearance remaining a mystery. Trying to keep my now all alone Olive company as much as I can.
ML, maybe she will, and maybe she won't. I have never had a duck stay away more than one night. She was strong willed and the now deceased (from natural causes) drake's fave. Maybe she is looking for him? (He passed 7 weeks ago)