The ones who disappeared sound like a predator... I lost two last week to something(likely a dog given the circumstances) the other one that flopped dead, ugh more puzzling but it's happened to me, I had a duck who i found dead with no obvious symptoms of anything. Nothing has ever happened since, and she had disappeared on us.
I have learned in my short poultry experience death comes hand n hand with owing them, some you can explain, others you won't. I enjoy them but this aspect leaves me less than completely thrilled with them, if you know what i mean? Sorry you lost them, i do understand how upsetting that is.