Ferrets? Possibly even just ferret bedding tossed about the place. I've posted before - my landlady where I used to stay had had rat problems for previous 30 odd years. I got three ferrets, used to exercise them in the yard, stables and sheds and they would sometimes go into the drains underground, under pallets, etc.
To my knowledge, my ferrets never made a kill there but the rats moved out. Her rat man stopped filling up bait stations, cat stopped bringing them in. No more chewed sacks. She swears it was the ferrets, having tried everything for decades.
BUT they will massacre hens if go awol and get into shed one night.
Agree that I would have thought something more protein based would be more attractive, raw fish, chicken bones? Maybe?
I have a few rats in the garden, but they've dug underneath the trap I put out - this is ok, because they're visiting it every night and I will shortly change it so they can't get bait without going inside.
I also have fenn trapsm not got those out yet - don't seem to have time with baby.