I'm surprised that your dogs didn't carry on if it were a fox. The M.O. is typical but the lack of your dogs having any reaction seems rather peculiar...
My guess would be a dog or cat. If your dogs go nuts over other dogs but pay no mind to cats prowling around...perhaps a cat is your culprit. Cats "on the hunt" are very quiet. I have to agree with the other posters that animals in the wild don't leave their supper behind.