It's not unusual for foxes to take poultry without leaving much sign. They are inclined to grab the hen and leg it. They will often go a good distance with the live bird before stopping to kill it.
I was driving home one evening, in the wee small hours, and met a fox leaving my neighbours farm with one of his black rock hens in its mouth. The road was icy, so I couldn't knock it over with the car, but I did cause it to drop the hen, which sat in the road till I picked it up. I took the hen home and, despite bruising to the breast, it recovered and was returned to my neighbour. A couple of days later, the fox was shot in my neighbours yard, eyeing up his remaining birds.
The fox seems to hold the bird by the breast/ribs which, I believe, has the effect of sort of paralizing it and suffocating at the same time. The remote pile of feathers, which are often found if the direction the fox took is known, are at the location where he put the hen down, cleared his mouth of feathers, and killed the hen before continuing on his way.