ow,, that's a hard one to figure out, rats and weasels like to tunnel and they can chew through almost anything, typically through they go for eggs and grain and kill at night, usually not large chickens. wieasels kill for fun andso do cats. A feral cat can do some damage too, and it will kill day or night and isn't afraid of domestic set ups.
I would drop some poison in the rat hole. REmove any hiding place near the coop and then really fortify the chicken area. It sounds like more than one predator, and you will have to eliminate it now that it found the restaurant. If you don't have time for vigilant sheparding, limit the time they spend outside to when you can watch them. with a good vantage point and apprropriate rifle. oh, açording to your local laws of course. second is traps, but not so effective.
another option is to provide yor chickens with cover, some place they can flee to and wwhere you can hear the alarm. dogs are wonderful deterents too. a border collie would work well with lllamas and chickens, they also hunt rats, and would like to spend all day watching their flocks. they are easy to train, think of the fun you could have hearding chickens with a dog, make foe some good video.
good luck with the defense,seems like as soon as I get one predator eliminated another takes it's place. this week it was either a coyote or a wolf, my husband shot at it and it dropped my hen but got away. Now the flock won't go across the road,finally. then a hawk. before that a cat,who is now dead,but spotted another cat too near us. It goes on and on.
the solution is to finish the perimeter fence And turn the dogs loose at dusk.Meanwhile vigilance and a strong coop.