Sorry to hear your sad news, Dizzycow

I do hope it doesn't happen again...
We have a repeating pattern of going quite a while with no losses, then losing one, then a few nights later another, and so on. Personally I have usually felt it was a fox - if s/he can lift one quietly and take it away to eat, there's no need to slaughter the whole flock, whereas if the hens are in an enclosed space that the fox has got into, they start panicking and flapping about, so the fox can hardly help itself but kill them all.
Before I lived on a farm I always had hens that got locked up safely at night, either with a light-sensitive electronic trap door or by human hand. Now I have the same problem as you - I want them to range freely during the day, so I'm helpless when they decide that the top of the Dutch Barn is a better bedroom than the nice lockable house I have provided for them. All is well until Foxy or Stoaty finds them...

Our only alternative is fox-proof runs, clipped wings, lockable fox- and stoat-proof house, no complete freedom.
We like to have them pecking about and being in the way, looking pretty, hiding their eggs, suddenly turning up with a brood... and have to accept that from time to time we will lose some birds.
Oh, and we too have lost eggs and young birds to rats - and to hedgehogs. A cat has moved into the barns now, so we are hoping that will assist in keeping rats at bay. I'm not sure what you can do about hedgehogs!