If at, or below, freezing during the night and you've completely checked over carcass for puncture wounds extending into thoracic/abdominal cavities?
Probably safe to use the organ meat (heart/liver/etc.) for gravy - boiled for a bit, but that's it.
I'd stake the remains of the carcass, out back, with a plastic 6ft. garden stake - Zip tie three fishing bells to the top of stake, turn up our shed/coop baby monitors so as to hear the jingling of the bells when old "sinner" fox showed up to finish its meal. Then I'd get one of the .22's, slide open the back door, have the wife spot light the fox and then I'd "slay" it.
Sorry for your loss and apologies for working in a few Xmas references into the preceding paragraph - but that's as "cheerful" a pose as you'll find this member straying towards when addressing the issue of this particular species of poultry killing vermin (if I'd had taken the trouble to skin & cure every fox we've harvested - the wife could be wearing a stole composed of nothing but the tails with a Red fur train extending so far behind that 10 maidens would have to be tasked to manage it (factored in the labor costs attendant to employing 10 maidens before outsourcing all Fox processing to the Turkey Vultures' feeding station).
As the other members noted; only way to be sure is to use the entire carcass as bait.
Edited by ivan3 - 12/26/15 at 3:15pm