I know the frustration having had 4 hens lost over a week or so from completely free ranging birds around a month ago now.
The ones he took were those that roamed up the field behind my house but he was right in by the barn for 2 of them and the third was taken outside my neighbour's kitchen window, the 4th I never found the feather patch to show where she was taken but she did favour a particular patch of paddock and pen and was forever about the tractor route in winter so that's my guess.
In my case I found it hard to decide whether to pen and effectively put them all in one place where they couldn't escape if the fox did get in, or leave them to free range where they might wander closer to him but if he went for one he'd not have the lot as the rest would get away in all directions. Under pressure from folk that demand doing something, regardless of the actual pros and cons, I opted for a compromise of penning from corn time 6pm to pellet time 6am on the basis that the one time I actually saw the fox it was 4am so there wouldn't be roaming hens up the field at that time. So far so good, no more lost, but I am not convinced it is good management on my part at all, far from it, I think I've lined up for one big desperate killing frenzy and may regret it one morning. And the ones that roamed furthest have gone so the homebodies are left and harder for him to spot in the jungle that is my garden compared to open pasture!
Most likely scenarios are that either the cubs are older and starting to demand less, or the farmers up the back have shot him - I mentioned my hen loss to one of them and he said another had had visits and did I know what time, so I told him. Maybe my info got the job done and I am not entirely sure I am glad as I can't blame a fox for feeding its cubs, I can't hate only be sad for my girls that went, each one with a name and a story, probably the wrong approach to keeping them in the first place!
Anyway, whatever you can do, an elec wire around their area or some limitation of their free ranging, I tend to think it's better than doing nothing and waiting for the inevitable. Not because he won't take them, he may, but because you won't feel quite so helpless while you have things to try and some sense of protecting them the best you can. Whether it's good enough or not, whether folk agree with you or not, is less important than feeling you have options and choosing from what you can see available to you. My personal opinion, no offence to anyone else's beliefs or practices intended.
Oh, I also have 3 determined broodys which come down for corn but 2 of them still manage to fly out and back to the hopeless last 2 eggs of a clutch with no hope of hatching now. One is a tiny bantam that has learned to do vertical takeoff to a 2x1 inch wide 6 foot high post at the gate area, I see her do it and am in awe of her skills. The other is the most determined commercial warren type broody ever and she has a squirrel like escape plan that evolves. Some days they won't come out of the gap in the round bales at all so whatever I do with the rest, they're vulnerable.
I don't know, but I tend tothink if they're that determined then whatever life they have they'll be happy doing it their own way even if it's objectively bl**dy minded and ridiculous to everyone else. I tend to be that way myself so can hardly criticise. You do what you think right, cos you're the one that may have to live with that being wrong, or enjoy it being right, or worry because it's still unresolved. All you can do really. Protect the best you can and accept the world isn't perfect, nor is anyone.