It was a cat. A half-grown wild kitten was in my coop when I went out to feed this afternoon. I had opened the pop door this morning on the off-chance they'd venture out. (they didn't - they don't like the cold and snow) The mystery is, how it got into the run to get in the coop. I have 2x4 welded wire all the way around it, and over the top. Apparently that's still big enough for a cat that size to get through. It wasn't doing any damage, just hanging out, laying in the straw, chickens wandering around totally oblivious to its presence. When I came in, it ran out the pop door and when I went out to see where it got in, it was on the top of the run. As far as I could tell, there weren't any gaps. Hmmm - Houdini cat?