I have lived here for 60 years and the only 2 i have ever seen her are the two i caught after they had killed a bunch of my birds the night before. here is a website that shows the habitat of the weasel in the US.
https://nhpbs.org/natureworks/longtailedweasel.htm
I dont know if this is your...
I aggree that something is getting in your coop. Are you sure there are no gaps or holes? A weasel can fit through a hole you might think only a mouse could.
I am so sorry about your birds.