I used to keep my hens in at my last place with electrified netting, run off a car battery - the hen house was in the middle, I never shut the pop hole at night.
It was next to a spring and foxes would come for miles around to drink and I never lost one to hens, ever. My neighbours, who had aviary-type houses, were losing them all the time to foxes digging under etc.
I watched foxes come past lots and saw the footprints around the perimiter in the snow. I have absolutely no doubt that the second I turned it off and went inside they would have been in and taken the lot.