I just went through the same thing. Unfortunately my DaphneDuck passed Sunday night. Like yours, really no symptoms. For only a handful of times over about one month, she wasn't wanting to eat mealworm treats in the evening, which is never passed up on, & just not seeming her usual self. She would stand around watching my other 8 eat treats and/or go sit down. I did see her eat food, she was drinking plenty, & pooping. Most days she seemed completely normal. Even the same day before she died, she was out swimming, foraging around with all the others. That evening, tho, she went into the pen, no mealwoms & soon after went into a dog igloo that I have in there. She did come back out a bit later, but that is ultimately where I found her in the morning. I could just feel that something was wrong, so I was going call Monday morning & get appt with vet.
I'm worried about my others in case of something contagious. I've been doing a lot of research but with no symptoms to go on it could be anything. I did read, however, about someone who had a similar situation and the vet prescribed a dewormer & antibiotics, but I don't know the outcome of any test results, but duck did seem to be doing better just a day later. I'm hesitant to treat everyone tho when it may not be necessary.
I will keep you posted if I learn anything new.