I know how it is, I just had 6 pullets killed by one fox, in 10 minutes two days ago. I had the door open so I could keep an eye on them and listen to them and the dogs could hear if anything was going on. My brother got home from the bus stop and they were all fine. 10 minutes later he yelled, FOX!!! And my sweet barred rock pullet was gone, along with 4 of my black sex links and a leghorn pullet was dying in my yard. The fox dropped her as we ran outside. We don't know how the dogs didn't hear the fox and none of the birds made any noise.
I am in the same boat as you so here is my advice. All your chickens need to go on lockdown immediately. Shut them in the coop or the run, somewhere safe. They may not like it but that fox will keep killing them.
Next you need to get rid of the fox. I suggest trapping it. Fox are extremely...foxy.. And cunning. I went to my local feed store and bought 2 live traps. One raccoon sized, the other was mink sized. I have rubber boots, a hunting jacket, thick pants, thick gloves, and scent killer spray. I sprayed down all the clothes with the scent killer. It's extremely important they can't smell human scent, and the scent killer I use is sold at hunting stores (gander mountain, dunhams, bass pro shops, and probably even
walmart) and I've been using it a while for deer and turkey hunting to mask scent. It works well for that so I trust it to spray the trap.
Next I took both of the traps near the place the chickens were killed. I set them up longways, the small one behind the big one. I covered them with leaves, sticks, brush, and dirt. I sprayed those down with scent killer too. I put a bantam in the small trap, and left the big one armed. I made sure the only way you can see the bantam, is looking through the larger trap. I only leave the bantam in there overnight with food, and luckily I have plenty of bantams, so I can rotate them without one getting too stressed. From what I've read/heard, this method has been successful for others catching fox. It's only been 2 days and I'm sure he's still snacking on the others he took.....but he will be back and I'm not letting my chickens out until I catch him. Remember, if it looks like a trap, the fox will know its a trap. Disguise it as best as you can and make it look natural as possible. I can't guarantee this will work, but this is the exact method I saw someone successfully trap a fox in a havahart trap, so there's a good shot. Good luck catching your foxes.