Jelmer,
        I wouldn't rule out the 2 legged predator!! I have had a run in with a couple and my big mean st. bernard, border collie mix now stays about 5 ft. from my pen as does the golden retriever! Do you have a dog?? It's just usually a rule of thumb that almost any predatory attack leaves behind some evidence especially a roo. Now a baby chick that's easy to cart off without a feather but a roo, try grabbing one that's on the run your gonna loose feathers and a predator would leave blood. 
      I had a guy come by wanted my chickens hubby said no they were my pets (this guy was BEATING on the door 6 am on a Saturday) 
		
		
	
	
  , later he came back again demanding I sell some to him, I said no. About an hour later the Mennonite boys next door came running and said he stopped them on their bikes and told them he'd buy all they could catch!!!
 They told him those were mine and he said oh he didn't know. Another lady caught a guy in her henhouse with a pillowcase throwing them in as fast as he could. Hard to believe but there are chicken thieves out there. Either way I would opt for getting a good dog, and maybe a shotgun, we have 4 and nothing bothers our babies. And we have everything from bear,bobcat, to coyote, fox, opossum, weasel, wild dogs, hawks, feral cats, owls, and 2 legged weasels!  
                                Hope it works out for you.
                                     Barb