Lilly is ok! She wasn't grabbed by a predator after all!
She was wedged into one of the concrete blocks that is the foundation of the coop! I don't know how she got in there but this morning I heard a kind of muffled peep sound coming from the ground under the edge of the coop and when I went into the coop I saw a tiny fluff of white feathers sticking out of the block. I managed to get her out and she seems ok - just hungry and thirsty! I am soooo relieved!