Something tripped a trap, but we didn't catch it. Trap reset.
Noticed that the hay where the chicks would normally be looks like a tiny hog came through and rooted through it. Followed a path in the hay to a small opening in where one wall meets another. Leads to the hay storage area. Went in there, followed path to behind hay bale. Found a musky smell and what looks unmistakably like fresh weasel poop. Whelp. Looks like you guys pegged the culprit. Not even my parents in all their years ever dealt with weasels, so none of us had any idea about the calling card kill style of those guys, but I've read up on it through the day and it all adds up.
Thanks for everyone's input. If we do manage to catch the little demon, I'll update again.